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GELE MathematicsPlane and Spherical TrigonometryMemory Anchors

Memory anchors and mnemonic tricks for Plane and Spherical Trigonometry. If you find yourself forgetting key facts from this chapter during GELE mocks, these anchors are your fix. Built for Professional Regulation Commission (PRC) — Board of Geodetic Engineering's question style and the time pressure of the GELE 2026.

Exam context

Professional Regulation Commission (PRC) — Board of Geodetic Engineering runs the Geodetic Engineer Licensure Examination on September 2026. Its Mathematics section sits under a "Core" weighting, and Plane and Spherical Trigonometry is the 2nd chapter in the 10-chapter GELE Mathematics rotation. The GELE passing mark is 70% weighted average, no sub-test below 50%, and the most recent 2026 paper drew about a meaningful share of questions from Mathematics.

Plane and Spherical Trigonometry - Memory Anchors

Memory techniques transform abstract trigonometric formulas into vivid, unforgettable mental images. Research in cognitive psychology (elaborative encoding) shows that associating new information with familiar stories, emotions, or sensory images increases long-term recall by up to 600%. For the PRC CE Board Exam, where trigonometry questions appear in both Mathematics and Surveying, internalizing these anchors means you can reconstruct any formula under exam pressure — even after a sleepless review night. Work through each anchor, visualize it fully, and test yourself using the recall triggers. The goal is not just to memorize — it is to OWN these formulas.

Anchors

Tags

  • formula
  • definition
  • right triangle
  • basic ratios

Topic

Right Triangle Trigonometry

Concept

SOH-CAH-TOA — Basic Trigonometric Ratios

Anchor Id

A1

Difficulty

easy

Memory Aid

Imagine a Filipino student named SONYA CAHAYAN-TOA. She is a surveyor holding a theodolite. SOH = Sine is Opposite over Hypotenuse. CAH = Cosine is Adjacent over Hypotenuse. TOA = Tangent is Opposite over Adjacent. Every time you see a right triangle, picture Sonya setting up her instrument — she always starts with the Opposite side first.

Anchor Type

acronym

Why It Works

The acronym SOH-CAH-TOA is a globally proven mnemonic. Adding a vivid Filipino character (Sonya) and connecting it to surveying (relevant to CE board) creates multi-layered encoding.

Example Usage

Q: A 50 m guy wire makes a 35° angle with the ground. Find the height of the tower. Think SOH: sin35° = h/50, so h = 50 × sin35° = 28.68 m.

Recall Trigger

Right triangle with an unknown side or angle → think 'SONYA CAHAYAN-TOA'

Tags

  • formula
  • identity
  • Pythagorean

Topic

Trigonometric Identities

Concept

Pythagorean Identity: sin²θ + cos²θ = 1

Anchor Id

A2

Difficulty

easy

Memory Aid

Think of SIN and COS as two basketball players on a team. No matter what angle you play at, their combined 'energy squares' always total 1 (a perfect full game). If sin²θ = 0.36, then cos²θ must be 0.64 — they always complete each other to make a whole. The TEAM TOTAL IS ALWAYS 1.

Anchor Type

analogy

Why It Works

The teamwork analogy anchors the idea of complementary values summing to a constant, making the identity feel logical rather than arbitrary.

Example Usage

If sinθ = 0.6, find cosθ. Team total = 1: cos²θ = 1 − 0.36 = 0.64, cosθ = 0.8.

Recall Trigger

Any identity problem → 'The basketball team total is 1'

Tags

  • formula
  • oblique triangle
  • law of sines

Topic

Law of Sines

Concept

Law of Sines: a/sinA = b/sinB = c/sinC

Anchor Id

A3

Difficulty

medium

Memory Aid

Remember: 'SIDE over SINE, match your PAIR.' Each side is always paired with the angle directly ACROSS from it (the opposite angle). Think of it as a dance: side 'a' dances only with angle 'A', side 'b' dances only with angle 'B'. They are always partners. The ratio of every couple is the same — the DANCE FLOOR RATIO.

Anchor Type

mnemonic

Why It Works

The dance partner metaphor emphasizes the opposite pairing rule, which is the single most important thing to remember about the Law of Sines.

Example Usage

Given a=10, A=30°, B=45°: b/sin45° = 10/sin30°, b = 10×sin45°/sin30° = 14.14.

Recall Trigger

Oblique triangle, two angle-side pairs known → 'Side-Sine dance partners'

Tags

  • formula
  • oblique triangle
  • law of cosines

Topic

Law of Cosines

Concept

Law of Cosines: c² = a² + b² − 2ab·cosC

Anchor Id

A4

Difficulty

medium

Memory Aid

Story: Engineer Carlo is applying the Pythagorean Theorem (c² = a² + b²) to an oblique triangle. His boss says, 'Carlo! You forgot the CORRECTION TERM!' The correction is −2ab·cosC. When C = 90°, cosC = 0 and the correction vanishes — that is why Pythagoras works only for right triangles. The Law of Cosines is Pythagoras with a built-in CORRECTION for non-right angles. Carlo never forgets his correction term again.

Anchor Type

micro_story

Why It Works

Framing the law of cosines as a 'corrected Pythagorean theorem' builds on prior knowledge and explains WHY the formula exists, making it easier to reconstruct from memory.

Example Usage

a=8, b=6, C=60°: c² = 64+36−2(8)(6)cos60° = 100−48 = 52, c = 7.21.

Recall Trigger

Two sides + included angle, or three sides given → 'Carlo's correction term −2ab·cosC'

Tags

  • formula
  • area
  • Heron's formula

Topic

Area of Triangle

Concept

Heron's Formula: A = √[s(s−a)(s−b)(s−c)], where s = (a+b+c)/2

Anchor Id

A5

Difficulty

medium

Memory Aid

Remember 'HERON SHOOTS FOUR ARROWS': s is the semi-perimeter (Half-perimeter), and inside the square root you have FOUR factors: s, (s−a), (s−b), (s−c). The word SEMI is critical — s is HALF the perimeter. Many students use full perimeter — that is the fatal mistake. Picture Heron the Greek hero pulling back a bow: he always cuts his string in HALF first (semi-perimeter).

Anchor Type

mnemonic

Why It Works

The visual of Heron cutting string in half dramatizes the semi-perimeter definition, and 'four arrows' helps recall the four factors under the radical.

Example Usage

Sides 5, 7, 9: s=(5+7+9)/2=10.5; A=√[10.5×5.5×3.5×1.5]=√[302.8125]=17.40 sq units.

Recall Trigger

Three sides given, find area → 'Heron cuts string in HALF first'

Tags

  • formula
  • double-angle
  • identity

Topic

Double-Angle Formulas

Concept

Double-Angle Formula: sin2θ = 2sinθcosθ

Anchor Id

A6

Difficulty

medium

Memory Aid

Rhyme: 'SINE DOUBLE, MULTIPLY BY TWO — SINE times COSINE, that will do!' sin2θ = 2·sinθ·cosθ. The key: there is a 2 in front, AND both sin and cos appear together — they always travel as a pair in this formula. No squares, no subtraction — just 2 × sin × cos.

Anchor Type

rhyme

Why It Works

Rhymes leverage phonological memory loops, making the formula pattern stick even when retrieved under stress.

Example Usage

Simplify 2sin30°cos30°: This IS sin2(30°) = sin60° = 0.866.

Recall Trigger

Any sin2θ term → hum 'Sine double, multiply by two'

Tags

  • formula
  • double-angle
  • identity
  • multiple forms

Topic

Double-Angle Formulas

Concept

Double-Angle Formula: cos2θ = cos²θ − sin²θ = 1−2sin²θ = 2cos²θ−1

Anchor Id

A7

Difficulty

hard

Memory Aid

COS DOUBLE has THREE FACES — memorize with '3 FACES OF COS DOUBLE': Face 1: cos²θ − sin²θ (cosine minus sine squared). Face 2: 1 − 2sin²θ (used when you want only sines). Face 3: 2cos²θ − 1 (used when you want only cosines). Think of a traffic light with 3 lights — you choose which face (which form) depending on what the problem needs to eliminate.

Anchor Type

chunking

Why It Works

Chunking the three forms as '3 faces' helps students recognize which version to deploy, preventing formula confusion under exam conditions.

Example Usage

If only sinθ terms allowed: use cos2θ = 1−2sin²θ. If cosθ=0.8: cos2θ = 2(0.64)−1 = 0.28.

Recall Trigger

cos2θ → '3 traffic light faces — pick the right color'

Tags

  • ambiguous case
  • SSA
  • law of sines
  • common pitfall

Topic

Law of Sines — Ambiguous Case

Concept

Ambiguous Case (SSA) in Law of Sines — Two Triangle Solutions Possible

Anchor Id

A8

Difficulty

hard

Memory Aid

Story: Engr. Ana receives a survey with two sides and a non-included angle (SSA). She draws one triangle — then realizes she can draw ANOTHER valid triangle on the other side of the baseline! Her supervisor says, 'Ana, SSA is AMBIGUOUS — always check if h < a < b (where h = b·sinA), because two triangles might exist!' The boarder who forgets this loses precious points. SSA = 'Sometimes Second Answer!'

Anchor Type

micro_story

Why It Works

The story personalizes the ambiguous case and the acronym SSA = 'Sometimes Second Answer' encodes the warning directly in the abbreviation.

Example Usage

If a=8, b=10, A=30°: h=10×sin30°=5. Since 5<8<10, two triangles exist. Solve for both B values: sinB=10sin30°/8=0.625, B=38.68° or B=141.32°.

Recall Trigger

Law of sines with SSA given → 'Sometimes Second Answer — draw both!'

Tags

  • formula
  • area
  • included angle

Topic

Area of Triangle

Concept

Area of Triangle: A = ½ab·sinC

Anchor Id

A9

Difficulty

easy

Memory Aid

Think of the parallelogram area (base × height). A triangle is HALF a parallelogram. Two sides form the parallelogram, and sinC captures the effective height. So area = HALF × side1 × side2 × sine of the INCLUDED ANGLE. Picture two sticks (a and b) opened like scissors at angle C — the area swept between them is ½ab·sinC.

Anchor Type

analogy

Why It Works

Connecting the formula to the well-known parallelogram area provides a geometric reason WHY the formula works, creating deeper encoding.

Example Usage

a=8, b=6, C=60°: A=½(8)(6)sin60°=½(48)(0.866)=20.78 sq m.

Recall Trigger

Two sides + included angle, find area → 'Scissors formula: ½ab·sinC'

Tags

  • formula
  • compound angle
  • identity
  • sign rules

Topic

Sum and Difference Formulas

Concept

Sum and Difference Formulas: sin(A±B) and cos(A±B)

Anchor Id

A10

Difficulty

hard

Memory Aid

Use the FOIL-like pattern with a SIGN RULE trick: For SIN: sin(A+B) = sinA·cosB + cosA·sinB. Notice SIN always pairs with COS (they cross-multiply). For COS: cos(A+B) = cosA·cosB − sinA·sinB. Notice COS pairs with COS and SIN pairs with SIN — but the sign FLIPS. Memory phrase: 'SINE CROSSES, COSINE STAYS, BUT SIGN CHANGES WHEN COS PLAYS.' The critical trick: sin formula keeps the SAME sign (+ stays +, − stays −), but cos formula FLIPS the sign (+ becomes −, − becomes +).

Anchor Type

mnemonic

Why It Works

The phrase highlights the most error-prone aspect (the sign flip in cosine formula), preventing the most common board exam mistake in this topic.

Example Usage

sin75° = sin(45°+30°) = sin45°cos30°+cos45°sin30° = (0.7071)(0.8660)+(0.7071)(0.5) = 0.9659.

Recall Trigger

Compound angle → 'Sine crosses, cosine stays, but sign changes when cos plays'

Tags

  • strategy
  • decision
  • law of sines
  • law of cosines

Topic

Triangle Solution Strategy

Concept

When to Use Law of Sines vs. Law of Cosines

Anchor Id

A11

Difficulty

medium

Memory Aid

Visualize TWO DOORS in the exam room: DOOR SINE has a label 'AAS / ASA / SSA' — angle-angle-side or angle-side-angle situations where you have matching angle-side pairs. DOOR COSINE has a label 'SAS / SSS' — when you have the included angle or all three sides. If you have a complete angle-side pair to START with, use Sines. If you are stuck with two sides and the angle between them (or all three sides), use Cosines. SINE = 'I have matching pairs.' COSINE = 'I have sides stuck together.'

Anchor Type

visual_association

Why It Works

The two-door visual forces a decision-making framework, which is exactly what students need during the exam to quickly classify which law applies.

Example Usage

Given b=12, A=40°, C=75°: angles known → use Law of Sines (Door Sine). Given a=5, b=7, C=110°: SAS → use Law of Cosines (Door Cosine).

Recall Trigger

Oblique triangle problem → 'Which door? Pairs = Sine, Stuck sides = Cosine'

Tags

  • formula
  • spherical triangle
  • law of sines

Topic

Spherical Trigonometry

Concept

Spherical Triangle Law of Sines: sinA/sina = sinB/sinb = sinC/sinc

Anchor Id

A12

Difficulty

hard

Memory Aid

In a PLANE triangle, Law of Sines uses the sides directly: a/sinA. In a SPHERICAL triangle, it is the same dance, but now both the ANGLE and the SIDE are wrapped inside SINE functions — because the 'sides' are arcs measured in degrees (angular measure). Imagine the flat triangle inflated onto a globe like a balloon: the formula 'inflates' too — both numerator and denominator become sines. Flat = side on top. Spherical = sin(side) on top.

Anchor Type

analogy

Why It Works

The balloon inflation analogy creates a mental image of the transition from plane to spherical, and the parallel structure of the two laws makes comparison natural.

Example Usage

Spherical triangle: sinA/sina = sinB/sinb. Given a=70°, A=80°, b=50°: sinB = sin50°×sin80°/sin70° = 0.789.

Recall Trigger

Spherical triangle → 'Inflate the triangle — both get wrapped in sine'

Tags

  • formula
  • spherical excess
  • area
  • spherical triangle

Topic

Spherical Trigonometry

Concept

Spherical Excess and Area of Spherical Triangle: E = (A+B+C)−180°; Area = πR²E/180°

Anchor Id

A13

Difficulty

hard

Memory Aid

Story: A surveyor mapping the Philippines on a globe discovers that his triangular survey region has angles summing to MORE than 180° — unlike flat triangles! The EXTRA amount over 180° is called SPHERICAL EXCESS E. The bigger the excess, the bigger the triangle on the globe. To get the area: multiply the excess (in degrees) by πR²/180°. Remember: 'EXCESS is the BONUS above 180°, and it PAYS as area on the sphere.'

Anchor Type

micro_story

Why It Works

The story grounds the abstract spherical excess concept in the familiar context of Philippine surveying, making it personally relevant and emotionally engaging.

Example Usage

Angles 95°, 85°, 100°: E=(95+85+100)−180=100°. R=6m: Area=π(36)(100)/180=62.83 sq m.

Recall Trigger

Spherical triangle area → 'Bonus above 180° pays as area'

Tags

  • pitfall
  • calculator
  • degree
  • radian

Topic

Common Board Exam Pitfalls

Concept

Degrees vs. Radians — Calculator Mode Pitfall

Anchor Id

A14

Difficulty

easy

Memory Aid

Board exam horror story: A reviewee solves a perfect setup but gets sin(π/6) = −0.0087 instead of 0.5 — because the calculator was in RADIAN mode with the angle entered as '30' (treated as 30 radians, not 30 degrees). He lost 2 points. After the exam, he taped a sticky note to his calculator: 'CHECK MODE BEFORE EVERY TRIG COMPUTATION.' Lesson: Before EVERY trig calculation, press MODE → confirm DEGREE or RADIAN. Make this a RITUAL.

Anchor Type

micro_story

Why It Works

The 'horror story' format creates an emotional warning memory. The specific wrong answer (−0.0087) makes it visceral and memorable.

Example Usage

sin30° in degree mode = 0.5 (correct). sin30 in radian mode = −0.9880 (WRONG). Always verify before submitting.

Recall Trigger

Any trig calculation → 'Check calculator MODE first — RITUAL!'

Tags

  • identity
  • reciprocal
  • definition

Topic

Reciprocal Identities

Concept

Reciprocal Identities: csc=1/sin, sec=1/cos, cot=1/tan

Anchor Id

A15

Difficulty

easy

Memory Aid

Use the CO-RECIPROCAL RULE: The 'co-' functions are reciprocals of the non-'co-' functions — but with a TWIST: csc (co-secant) is 1/sin (NOT 1/cos). Avoid this trap with: 'COsecant = 1/Sine, SECant = 1/Cosine.' Easy memory: 'CSC and SIN are enemies — they are reciprocals. SEC and COS are enemies — they are reciprocals.' Or use: C-S-S-C (Cosecant-Sine, Secant-Cosine are pairs). For COT: it is simply 1/tan or cosθ/sinθ.

Anchor Type

acronym

Why It Works

The 'enemies' framing highlights the counterintuitive pairing (csc↔sin, not csc↔cos) which is the most common confusion in this area.

Example Usage

If sinθ = 3/5, then cscθ = 5/3. If cosθ = 4/5, then secθ = 5/4.

Recall Trigger

csc, sec, cot → 'CSC-SIN enemies, SEC-COS enemies'

Tags

  • angle of elevation
  • angle of depression
  • applied
  • setup

Topic

Applied Trigonometry

Concept

Angle of Elevation vs. Angle of Depression — Correct Setup

Anchor Id

A16

Difficulty

easy

Memory Aid

ELEVATION = look UP from horizontal (eyes go UP like elevating). DEPRESSION = look DOWN from horizontal (eyes go DOWN like depressed). Critical: both angles are measured from the HORIZONTAL, NOT from the vertical. Visualize a surveyor standing on flat ground: elevation angle is between his level line of sight and the line going UP to the target. In both cases, the angle is at the OBSERVER, and the tangent formula gives tan(angle) = vertical/horizontal distance.

Anchor Type

visual_association

Why It Works

Connecting the words 'elevation' (up) and 'depression' (down) to their physical meaning eliminates the most common setup error in applied trig problems.

Example Usage

From 50 m away, elevation = 30°: h = 50·tan30° = 28.87 m. From tower top, depression to boat = 45°, tower h=30m: horizontal = 30/tan45° = 30 m.

Recall Trigger

Tower/building height problem → 'ELEVATION up, DEPRESSION down, both from HORIZONTAL'

Tags

  • identity
  • Pythagorean
  • derivation
  • family

Topic

Pythagorean Identities

Concept

1 + tan²θ = sec²θ and 1 + cot²θ = csc²θ

Anchor Id

A17

Difficulty

medium

Memory Aid

Remember the PYTHAGOREAN FAMILY of three identities as a POWER TRIO: Member 1 (Basic): sin²θ + cos²θ = 1. Divide everything by cos²θ → Member 2 (Tangent family): tan²θ + 1 = sec²θ. Divide Member 1 by sin²θ → Member 3 (Cotangent family): 1 + cot²θ = csc²θ. The TRICK: You only need to memorize Member 1. The other two are derived by DIVISION. On the exam, if you forget Members 2 or 3, just derive them in 10 seconds from sin²+cos²=1.

Anchor Type

chunking

Why It Works

Showing that all three identities come from one source (sin²+cos²=1) reduces memorization load — students only need to remember ONE formula and a simple derivation trick.

Example Usage

Simplify sec²θ − tan²θ: Using identity, sec²θ = 1+tan²θ, so sec²θ−tan²θ = 1.

Recall Trigger

sec² or csc² in a problem → 'Power Trio — divide the basic identity'

Tags

  • formula
  • spherical triangle
  • law of cosines

Topic

Spherical Trigonometry

Concept

Spherical Law of Cosines: cos a = cos b·cos c + sin b·sin c·cos A

Anchor Id

A18

Difficulty

hard

Memory Aid

Compare with the plane law of cosines: c² = a² + b² − 2ab·cosC. In the spherical version, EVERYTHING inflates into cosines and sines: the 'squared' sides become cos(side), the subtraction becomes addition, and the 2ab term becomes sin b·sin c. Think of it as the 'DOUBLE INFLATION' formula: inflate sides into cosines, inflate the cross-product term into sines. Also note: the plane formula SUBTRACTS, but the spherical formula ADDS — a common sign mistake.

Anchor Type

analogy

Why It Works

The structural parallel with the plane formula makes the spherical version feel less foreign, and the sign difference (+ vs −) is explicitly flagged as a pitfall.

Example Usage

Spherical triangle b=60°, c=50°, A=70°: cos a = cos60°·cos50°+sin60°·sin50°·cos70° = 0.5(0.6428)+(0.8660)(0.7660)(0.3420) = 0.3214+0.2270 = 0.5484, a = 56.8°.

Recall Trigger

Spherical triangle, SAS case → 'Double inflation — cos·cos + sin·sin·cosA'

Tags

  • Heron's formula
  • semi-perimeter
  • pitfall
  • area

Topic

Area of Triangle

Concept

Semi-perimeter s = (a+b+c)/2 in Heron's Formula

Anchor Id

A19

Difficulty

easy

Memory Aid

Quick rhyme: 'S is SEMI, not the SUM — divide by TWO before you're done!' The letter 's' stands for semi-perimeter. If you use the full perimeter, your answer will be massively wrong. Picture the letter 's' as a HALF-circle — it literally looks like half of something, reinforcing that s is HALF of the full perimeter.

Anchor Type

rhyme

Why It Works

The rhyme + visual (s as half-circle) creates two independent memory hooks for the same fact, doubling the chance of correct recall.

Example Usage

Sides 3, 4, 5: s = (3+4+5)/2 = 6 (NOT 12). A = √[6(3)(2)(1)] = √36 = 6 sq units.

Recall Trigger

Heron's formula → 'S is SEMI — looks like half a circle'

Tags

  • ASTC
  • quadrant
  • reference angle
  • signs

Topic

Quadrant Signs and Reference Angles

Concept

Reference Angles and Signs in Each Quadrant (ASTC Rule)

Anchor Id

A20

Difficulty

medium

Memory Aid

Philippines mnemonic: 'ANG SAYA TUWING CHRISTMAS!' — A-S-T-C: Quadrant I (All positive), Quadrant II (Sine positive), Quadrant III (Tangent positive), Quadrant IV (Cosine positive). Filipino version: 'All Students Take Calculus' or the Philippine twist 'Ang Saya Tuwing Christmas' (It's Fun Every Christmas). When a negative angle or angle >90° appears, find its reference angle, then apply the quadrant sign.

Anchor Type

acronym

Why It Works

The Filipino phrase 'Ang Saya Tuwing Christmas' is culturally resonant and creates a strong emotional memory hook through the joy of Christmas — a universal Filipino experience.

Example Usage

Find sin150°: Reference angle = 30°, Quadrant II (Sine positive): sin150° = +sin30° = +0.5. Find cos210°: Reference = 30°, Quadrant III (only Tan positive): cos210° = −cos30° = −0.866.

Recall Trigger

Angle in any quadrant → hum 'Ang Saya Tuwing Christmas — ASTC'

Revision Game

Sine (sinθ = opposite/hypotenuse)

Clue

I am the ratio of the side opposite your angle to the longest side of your right triangle. I am the first letter of SOH. Who am I?

Memory Link

A1 — SONYA CAHAYAN-TOA (SOH: Sine = Opposite over Hypotenuse)

Law of Cosines: c² = a²+b²−2ab·cosC

Clue

Engineer Carlo applied me to fix the Pythagorean theorem for non-right triangles. I subtract a special correction term. When the angle is 90°, my correction term vanishes. What formula am I?

Memory Link

A4 — Carlo's Correction Term story

cos2θ (double-angle cosine formula)

Clue

I have THREE FACES. My first face shows cos²θ−sin²θ. My second face shows 1−2sin²θ. My third face shows 2cos²θ−1. Who am I?

Memory Link

A7 — Three Traffic Light Faces of Cos Double

Heron's Formula: A = √[s(s−a)(s−b)(s−c)]

Clue

I am the Greek hero who shoots four arrows under a square root sign, but only after cutting the perimeter in HALF. I compute triangle area from three sides. Who am I?

Memory Link

A5 — Heron Shoots Four Arrows; A19 — S is SEMI rhyme

Spherical Excess: E = (A+B+C)−180°

Clue

Filipino surveyors doing geodesy on a globe use me. I measure the BONUS that a spherical triangle's angles exceed 180°. What am I called?

Memory Link

A13 — Bonus Pays Area story

SSA — The Ambiguous Case of the Law of Sines

Clue

In the Law of Sines, I am the dangerous case where two triangles might exist from the same given data. My three letters stand for Side-Side-Angle. I am also abbreviated as 'Sometimes Second Answer.' What case am I?

Memory Link

A8 — Engr. Ana's two-triangle discovery story

ASTC Rule — All, Sine, Tangent, Cosine are positive in Quadrants I, II, III, IV respectively

Clue

I live in all four quadrants, but in each quadrant only some trig functions are positive. In the Philippines, I am remembered by the phrase 'Ang Saya Tuwing Christmas.' What rule am I?

Memory Link

A20 — Ang Saya Tuwing Christmas mnemonic

Calculator MODE setting (Degree vs. Radian)

Clue

A reviewee lost 2 points on the board exam because I was set to RADIAN when the problem needed DEGREES. I am a tiny button on the calculator that changed everything. What am I?

Memory Link

A14 — Calculator Horror Story

Formula Mnemonics

Formula

sinθ = opp/hyp, cosθ = adj/hyp, tanθ = opp/adj

Mnemonic

SOH-CAH-TOA: Think of surveyor SONYA CAHAYAN-TOA. She measures opposite (SOH), adjacent (CAH), then the ratio (TOA).

When To Use

Always in a RIGHT triangle when you know an acute angle and need to find sides, or know sides and need to find the angle.

What Each Part Means

SOH: Sine = Opposite ÷ Hypotenuse. CAH: Cosine = Adjacent ÷ Hypotenuse. TOA: Tangent = Opposite ÷ Adjacent.

Formula

sin²θ + cos²θ = 1

Mnemonic

Basketball team total: sin and cos always sum their squares to 1 — they complete the perfect game together.

When To Use

Any time you need to find one trig function given another, or simplify expressions involving sin² + cos².

What Each Part Means

sin²θ = sin of angle, squared. cos²θ = cos of angle, squared. Together they ALWAYS equal exactly 1.

Formula

a/sinA = b/sinB = c/sinC (Law of Sines)

Mnemonic

DANCE PARTNERS: side 'a' dances only with angle 'A' across from it. Every couple has the same dance ratio.

When To Use

Given AAS, ASA, or SSA (careful — ambiguous case!). You must have at least one complete angle-side pair.

What Each Part Means

a, b, c = sides of oblique triangle. A, B, C = angles OPPOSITE to those respective sides. Each fraction equals the circumdiameter 2R.

Formula

c² = a² + b² − 2ab·cosC (Law of Cosines)

Mnemonic

Carlo's Correction: Pythagorean theorem with a CORRECTION TERM (−2ab·cosC). When C=90°, cosC=0, correction vanishes, Pythagoras returns.

When To Use

Given SAS (two sides + included angle) or SSS (all three sides). No complete angle-side pair available for Law of Sines.

What Each Part Means

c = side opposite angle C. a, b = the other two sides. C = included angle between sides a and b. −2ab·cosC corrects for the non-right angle.

Formula

A = ½ab·sinC

Mnemonic

SCISSORS FORMULA: two sticks (a and b) opened at angle C sweep out half a parallelogram.

When To Use

Two sides and the INCLUDED angle are known. If angle is not included between those two sides, use different approach.

What Each Part Means

a, b = any two sides of the triangle. C = the INCLUDED angle between those two sides (the angle between them, not any other angle). ½ = because triangle is half a parallelogram.

Formula

A = √[s(s−a)(s−b)(s−c)], s=(a+b+c)/2 (Heron's Formula)

Mnemonic

HERON SHOOTS FOUR ARROWS. s is SEMI-perimeter (half). Four factors under root: s, (s−a), (s−b), (s−c).

When To Use

Three sides given (SSS), no angles known. Use when neither ½ab·sinC nor the scissors formula applies directly.

What Each Part Means

s = semi-perimeter = HALF of (a+b+c). (s−a), (s−b), (s−c) = how much each side is 'short' of the semi-perimeter. The product under the root must be positive — triangle inequality check.

Formula

sin2θ = 2sinθcosθ

Mnemonic

SINE DOUBLE: multiply by 2, bring in BOTH sin and cos. No squares — just 2 × sin × cos.

When To Use

When you see sin(2θ) or need to simplify products like 2sinθcosθ into a single trig function.

What Each Part Means

2θ = double the angle. The result equals 2 times the product of sinθ and cosθ. The '2' is always present.

Formula

cos2θ = cos²θ−sin²θ = 1−2sin²θ = 2cos²θ−1

Mnemonic

THREE FACES OF COS DOUBLE: Traffic light — Red (cos²−sin²), Yellow (1−2sin²), Green (2cos²−1). Choose the face that eliminates the unwanted trig function.

When To Use

When simplifying integrals, proving identities, or solving equations involving cos(2θ). Choose the form based on what other terms appear.

What Each Part Means

Face 1: Both sin and cos present — use cos²−sin². Face 2: Only sin in final answer needed — use 1−2sin²θ. Face 3: Only cos in final answer needed — use 2cos²θ−1.

Formula

sin(A±B) = sinA·cosB ± cosA·sinB

Mnemonic

SINE CROSSES: sin and cos cross-multiply each other. Sign stays SAME (+ stays +, − stays −). Both terms have mixed sin-cos pairs.

When To Use

Finding exact values of angles like 75° (=45°+30°), 15° (=45°−30°). Also for sum-to-product and product-to-sum conversions.

What Each Part Means

sinA·cosB = first angle's sine × second angle's cosine. cosA·sinB = first angle's cosine × second angle's sine. The ± sign in the result MATCHES the ± in the argument.

Formula

cos(A±B) = cosA·cosB ∓ sinA·sinB

Mnemonic

COSINE STAYS, SIGN FLIPS: cos stays with cos, sin stays with sin — but the sign in the RESULT is the OPPOSITE of the sign in the argument (+ gives −, − gives +).

When To Use

Same as sin compound angle — exact angle values, identity proofs. The sign flip is the critical difference from the sine formula.

What Each Part Means

cosA·cosB = both cosines multiply. sinA·sinB = both sines multiply. Critical: if formula has (A+B), result subtracts (∓); if (A−B), result adds.

Formula

E = (A+B+C)−180°; Area = πR²E/180° (Spherical Triangle)

Mnemonic

BONUS PAYS AREA: Spherical Excess E = bonus above 180°. Area = πR² × (bonus in degrees)/180°.

When To Use

Finding area of a spherical triangle given its three angles and sphere radius. Common in geodesy and astronomy problems.

What Each Part Means

A, B, C = interior angles of spherical triangle (each >0°, sum >180°). E = excess over 180°, always positive. R = radius of the sphere. πR²E/180° converts angular excess to physical area.

Quick Recall Chains

Chain Title

Triangle Solution Cases — Which Law to Use

Recall Test

Q: A triangle has sides a=5, b=8, and angle A=30°. Which door? Answer: SSA → Door Sine (but check ambiguous case — is there a second triangle?).

Memory Chain

Think of TWO DOORS in the exam room. DOOR SINE (colored blue): enters if you have angle-side PAIRS — AAS, ASA, SSA. You need at least one matching pair to do the sine dance. DOOR COSINE (colored red): enters if sides are STUCK together — SAS (two sides with angle between them) or SSS (all sides, no angles). Special warning sticky on Door Sine: 'SSA — check for two solutions!' Walk through the correct door first, then solve.

Items To Remember

  • AAS → Law of Sines
  • ASA → Law of Sines
  • SSA → Law of Sines (check ambiguous case)
  • SAS → Law of Cosines
  • SSS → Law of Cosines or Heron's

Chain Title

Steps to Solve a Triangle Using Law of Cosines (SAS Case)

Recall Test

Q: Given a=8, b=6, C=60°. Walk through all 6 steps. Answer: c²=52, c=7.21; sinA/8=sin60°/7.21 → A=73.2°; B=180−60−73.2=46.8°; check 60+73.2+46.8=180°. ✓

Memory Chain

Carlo (our correction-term engineer) follows a 6-step checklist: WRITE the formula → SUBSTITUTE → COMPUTE c → SWITCH to Sines for angles → SUBTRACT for last angle → VERIFY sum is 180°. He never skips the verification — that saved him 3 points on the mock board.

Items To Remember

  • Step 1: Write c² = a²+b²−2ab·cosC
  • Step 2: Substitute known values
  • Step 3: Compute c (take square root)
  • Step 4: Use Law of Sines to find a second angle
  • Step 5: Find third angle by subtraction (A+B+C=180°)
  • Step 6: Verify — sum of angles must equal 180°

Chain Title

Pythagorean Identity Family — Derivation Chain

Recall Test

Q: Derive 1+cot²θ=csc²θ from the basic identity. Answer: sin²θ+cos²θ=1; divide through by sin²θ: 1+cot²θ=csc²θ. ✓

Memory Chain

The POWER TRIO starts as one. Divide by cos² → tan² joins sec². Divide by sin² → cot² joins csc². Three identities, ONE source. If you forget identities 2 or 3, divide identity 1 in 10 seconds.

Items To Remember

  • Start: sin²θ+cos²θ=1
  • Divide by cos²θ: tan²θ+1=sec²θ
  • Divide by sin²θ: 1+cot²θ=csc²θ

Chain Title

ASTC Quadrant Sign Rule

Recall Test

Q: What is cos(240°)? Answer: 240°−180°=60° reference angle, Quadrant III (Tangent only positive, so Cosine is NEGATIVE): cos(240°)=−cos60°=−0.5.

Memory Chain

ANG SAYA TUWING CHRISTMAS! A = All (Q1), S = Sine (Q2), T = Tangent (Q3), C = Cosine (Q4). Move counter-clockwise from Q1 and chant: All → Sine → Tangent → Cosine. Any angle > 90° — find reference angle, apply ASTC sign.

Items To Remember

  • Quadrant I: All positive (sin, cos, tan)
  • Quadrant II: Sine positive only
  • Quadrant III: Tangent positive only
  • Quadrant IV: Cosine positive only

Chain Title

Heron's Formula Application Steps

Recall Test

Q: Find area of triangle with sides 5, 7, 9. Answer: s=10.5; (5.5)(3.5)(1.5)=28.875; 10.5×28.875=303.19; √303.19=17.41 sq units.

Memory Chain

Heron's FOUR-STEP ARROW: HALF first (s=semi) → SUBTRACT each side → MULTIPLY all four → ROOT it. Remember: HALF → SUBTRACT → MULTIPLY → ROOT. If product under root is negative, the triangle is impossible (triangle inequality violated).

Items To Remember

  • Step 1: Compute s = (a+b+c)/2 (SEMI-perimeter)
  • Step 2: Compute (s−a), (s−b), (s−c)
  • Step 3: Multiply: s(s−a)(s−b)(s−c)
  • Step 4: Take square root for area
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