LET Secondary Mathematics — Logic and Mathematics in the Modern WorldSummary

A declarative sentence that is either true or false, but not both. It has a truth value (T or F). Examples: 'Manila is the capital of the Philippines' (true), 'The Earth is flat' (false). Non-examples: 'Close the door!' (command—not a statement), 'What time is it?' (question—not a statement). A simple statement contains one idea; a compound statement joins simple statements with logical connectives.

The truth value of a statement is either True (T) or False (F). In logic, every statement has exactly one truth value. Compound statements' truth values depend on the truth values of their components and the connectives that join them.

Negation reverses the truth value of a statement. If p is true, ¬p is false; if p is false, ¬p is true. Example: If p = 'It is raining,' then ¬p = 'It is not raining.' The truth table has only two rows: (T → F, F → T). Common mistake: Students confuse negation with antonyms; the negation of 'hot' is 'not hot,' not necessarily 'cold.'

A conjunction p ∧ q is true only when both p and q are true. Truth table: T∧T=T; T∧F=F; F∧T=F; F∧F=F. Example: 'The room is clean AND quiet' is true only if both conditions hold. If either is false, the whole statement is false.

A disjunction p ∨ q is true when at least one of p or q is true. Truth table: T∨T=T; T∨F=T; F∨T=T; F∨F=F. Example: 'You can have ice cream OR cake' means at least one choice is available (possibly both). The disjunction is false only when both parts are false.

A conditional p → q ('if p then q') has p as the hypothesis (antecedent) and q as the conclusion (consequent). The conditional is false only when p is true and q is false; all other combinations yield true. Truth table: T→T=T; T→F=F; F→T=T; F→F=T. Example: 'If it rains, then the ground is wet.' This statement fails to be true only when it rains (p true) but the ground remains dry (q false)—a clear contradiction.

A biconditional p ↔ q is true when p and q have the same truth value (both true or both false). Truth table: T↔T=T; T↔F=F; F↔T=F; F↔F=T. Example: 'A triangle is equilateral if and only if all its sides are equal' means the two conditions are equivalent; one is true exactly when the other is.

From a conditional p → q, the contrapositive is ¬q → ¬p (swap the parts and negate each). Example: Original: 'If a number is divisible by 6, then it is divisible by 3.' Contrapositive: 'If a number is not divisible by 3, then it is not divisible by 6.' The contrapositive is logically equivalent to the original—they always have the same truth value. This is the only variation that preserves logical equivalence.

From p → q: The converse is q → p (swap without negating). Example: Original 'If the ground is wet, then it rained'; converse 'If it rained, then the ground is wet.' The inverse is ¬p → ¬q (negate without swapping). Example: 'If it did not rain, then the ground is not wet.' The converse and inverse are logically equivalent to each other but not to the original. A true original can have a false converse or inverse.

Two fundamental rules for negating compound statements: (1) ¬(p ∧ q) ≡ ¬p ∨ ¬q (negating an 'and' yields an 'or' with both parts negated), and (2) ¬(p ∨ q) ≡ ¬p ∧ ¬q (negating an 'or' yields an 'and' with both parts negated). Example: The negation of 'The room is clean and quiet' is 'The room is not clean or it is not quiet.' Common error: Students incorrectly negate to 'The room is not clean and not quiet.'

A statement that is always true regardless of the truth values of its component parts. Example: p ∨ ¬p (either a statement is true or its negation is true; one of these must hold). The truth table has T in the final column for all rows. Contrasts with a contradiction, which is always false.

A statement that is always false regardless of the truth values of its parts. Example: p ∧ ¬p (a statement cannot be both true and false). The truth table has F in the final column for all rows.

Modus ponens is a valid argument form: (1) If p then q (premise), (2) p is true (premise), therefore (3) q must be true (conclusion). The structure guarantees that if both premises are true, the conclusion is certain. Example: (1) If a pupil studies, they will pass. (2) Maria studied. Therefore, (3) Maria will pass. The logic is airtight.

Modus tollens is a valid argument form: (1) If p then q (premise), (2) q is false (premise), therefore (3) p must be false (conclusion). Example: (1) If it rains, the ground is wet. (2) The ground is not wet. Therefore, (3) it did not rain. Denying the conclusion forces a denial of the antecedent.

A common logical fallacy: (1) If p then q (premise), (2) q is true (premise), therefore (3) p must be true (conclusion). This reasoning is invalid. Example: (1) If it rains, the ground is wet. (2) The ground is wet. (3) Therefore, it rained. But the ground could be wet from sprinklers, not rain. Affirming q does not guarantee p.

Another common fallacy: (1) If p then q (premise), (2) p is false (premise), therefore (3) q must be false (conclusion). This reasoning is invalid. Example: (1) If a student studies, they pass. (2) John did not study. (3) Therefore, John failed. But John might have passed anyway through prior knowledge or other means. Denying p does not force the denial of q.

Inductive reasoning moves from specific observations to a general conclusion (or pattern). It is probabilistic, not certain. Example: Observing that 2, 4, 6, 8 are all even and concluding 'all even numbers follow a pattern' is inductive. Strengths: It helps discover rules and patterns. Weaknesses: A single counterexample disproves an inductive claim. Example: If the sequence were 2, 4, 6, 8, 15 (breaking the pattern), the rule fails. Inductive reasoning is how scientists and learners form hypotheses.

Deductive reasoning moves from general premises to a specific, certain conclusion. If the premises are true and the logic is valid, the conclusion must be true. Example: (1) All right angles measure 90°. (2) Angle A is a right angle. Therefore, (3) Angle A measures 90°. The reasoning is airtight. Deductive reasoning is used in mathematical proofs and formal justifications.

An arithmetic sequence (or arithmetic progression) is a sequence where a constant difference d is added to each term to get the next. Example: 3, 7, 11, 15, ... has d = 4. The nth term formula is aₙ = a₁ + (n - 1)d, where a₁ is the first term. Example: For the sequence 3, 7, 11, 15, ..., find a₁₀: a₁₀ = 3 + (10 - 1)(4) = 3 + 36 = 39. The sum of the first n terms is Sₙ = (n/2)(a₁ + aₙ) or Sₙ = (n/2)[2a₁ + (n - 1)d].

A geometric sequence (or geometric progression) is a sequence where a constant ratio r is multiplied to each term to get the next. Example: 2, 6, 18, 54, ... has r = 3. The nth term formula is aₙ = a₁ × r^(n-1). Example: For the sequence 2, 6, 18, 54, ..., find a₆: a₆ = 2 × 3^(6-1) = 2 × 3^5 = 2 × 243 = 486. The sum of the first n terms (if r ≠ 1) is Sₙ = a₁(1 - r^n)/(1 - r).

The Fibonacci sequence is defined by the rule: each term is the sum of the two preceding terms. Starting with 1, 1, the sequence is 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ... Mathematically: F₁ = 1, F₂ = 1, and Fₙ = Fₙ₋₁ + Fₙ₋₂ for n > 2. The Fibonacci sequence appears throughout nature: the spirals of a pineapple, the arrangement of seeds in a sunflower, the branching of trees, and the spiral of a nautilus shell.

The golden ratio, denoted φ (phi), is approximately 1.618. It arises as the limit of the ratio of consecutive Fibonacci numbers: as n increases, Fₙ₊₁ / Fₙ approaches φ. Mathematically, φ = (1 + √5) / 2 ≈ 1.618. The golden ratio appears in proportions considered aesthetically pleasing in art, architecture (the Parthenon), and nature (human body proportions, flower petals).

Square numbers are numbers of the form n², where n is a positive integer: 1² = 1, 2² = 4, 3² = 9, 4² = 16, 5² = 25, 6² = 36, 7² = 49, 8² = 64, 9² = 81, 10² = 100. The sequence is 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, ... These numbers represent the area of a square with side length n.

Triangular numbers are sums of consecutive positive integers: 1, 1+2=3, 1+2+3=6, 1+2+3+4=10, 1+2+3+4+5=15, ... The nth triangular number is Tₙ = 1 + 2 + 3 + ... + n = n(n + 1)/2. The sequence is 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ... These numbers can be arranged in a triangular dot pattern.

Simple interest is interest calculated only on the principal (original amount) and does not compound. The formula is I = P × R × T, where I is interest, P is principal, R is the annual interest rate (as a decimal), and T is time in years. Example: A teacher invests ₱5,000 at 8% annual interest for 2 years. Interest = 5,000 × 0.08 × 2 = ₱800. Total amount = principal + interest = 5,000 + 800 = ₱5,800. Simple interest grows linearly.

Compound interest is interest earned on the principal and on accumulated interest; it compounds over periods. The formula is A = P(1 + r)^n, where A is the final amount, P is principal, r is the interest rate per period (as a decimal), and n is the number of periods. Example: ₱10,000 invested at 5% annual compound interest for 2 years yields A = 10,000 × (1.05)² = 10,000 × 1.1025 = ₱11,025. Interest earned = ₱1,025. Compound interest grows exponentially, faster than simple interest.

A check digit is a digit added to a number (like an ISBN, barcode, or bank card number) to detect errors. It is computed using modular arithmetic—working with remainders. For example, ISBNs use a check digit so that if one digit is mistyped, the total sum modulo 11 will not equal the expected value, triggering an error message. This quiet mathematics guards everyday transactions: when you enter a credit card number online and it is rejected, a check-digit algorithm likely detected a mistyped digit.

A set is a collection of distinct objects (elements). Sets are written in curly braces: A = {1, 2, 3, 4}. Key operations: (1) Union (A ∪ B): all elements in A or B (or both). Example: {1, 2, 3} ∪ {3, 4, 5} = {1, 2, 3, 4, 5}. (2) Intersection (A ∩ B): elements in both A and B. Example: {1, 2, 3} ∩ {3, 4, 5} = {3}. (3) Complement (A'): elements not in A (relative to a universal set). A subset has all its elements inside another set; the empty set ∅ has no elements.

Venn diagrams use overlapping circles or other shapes to visualize sets and their relationships. Overlapping regions show intersections; non-overlapping regions show elements unique to each set. Example: For sets A = {1, 2, 3, 4} and B = {3, 4, 5, 6}, a Venn diagram has two overlapping circles; the overlap contains {3, 4} (A ∩ B), the left-only region contains {1, 2} (A - B), and the right-only region contains {5, 6} (B - A).

Statistics and data analysis are mathematics applied to the real world. They turn raw numbers into summaries (mean, median, mode), patterns (graphs), and predictions (trends). Example: A teacher collects test scores from 30 pupils, creates a histogram to visualize the distribution, and computes the mean to gauge overall class performance. Modern applications include forecasting disease spread, predicting election results, and analyzing climate patterns.

Coding and cryptography use mathematics to encode information securely. Simple examples include the Caesar cipher (shift letters by a fixed number) and more modern methods like RSA encryption (based on the difficulty of factoring large numbers). Check digits (discussed above) are a form of coding. Modular arithmetic underpins many coding schemes.

All mathematical proofs and reasoning rest on logical principles. Inductive reasoning discovers patterns (hypotheses); deductive reasoning proves they hold universally. A valid argument (modus ponens or modus tollens) guarantees certainty from true premises. The contrapositive, De Morgan's Laws, and truth tables are tools teachers use to help students construct and evaluate sound arguments.

Conditional statements (if-then) are central to mathematics. Mathematical theorems are often conditionals: 'If a quadrilateral is a rectangle, then its opposite sides are equal.' The contrapositive is a powerful proof technique: to prove p → q, one can prove ¬q → ¬p instead (often easier). The converse is typically not true, a common source of student error.

Good teaching oscillates between induction and deduction. Teachers pose patterns inductively ('What comes next?') to engage pupils in discovery, then verify the pattern deductively using axioms and logical steps. This mirrors how mathematics develops historically and cognitively: conjecture (induction) then proof (deduction).

Arithmetic and geometric sequences are rules expressed in closed form (formulas). The Fibonacci sequence, though recursive (defined by a rule relating previous terms), also appears as a pattern in nature. Recognizing the type of sequence (arithmetic, geometric, or other) is the first step to solving sequence problems—a decision point on the LET.

The ratio of consecutive Fibonacci numbers approaches the golden ratio φ ≈ 1.618. This ratio appears in nature (spirals, petals, proportions) and in human-designed aesthetics (art, architecture). The connection illustrates that mathematics is not abstract but embedded in the beautiful, natural world.

Compound interest follows the geometric sequence model: each period, the amount is multiplied by (1 + r), where r is the rate. Over n periods, the amount grows exponentially. This connects sequences, exponential functions, and personal finance—a core modern-world application.

Sets and their operations (union, intersection) are visualized in Venn diagrams. The formula n(A ∪ B) = n(A) + n(B) - n(A ∩ B) counts without double-counting. This principle solves real-world problems: 'In a class, how many pupils like at least one of two subjects?' Set theory and logic are twin languages describing the same structure.

Check digits (ISBNs, card numbers, barcodes) use modular arithmetic to encode redundancy. If one digit is mistyped, the check-digit algorithm detects the error. This is practical cryptography: mathematics quietly protecting everyday digital life.

Statistics (mean, median, graphs) and data interpretation are mathematics applied to real-world information. Teachers use data to guide classroom decisions; society uses data to forecast and manage risks. Data literacy—interpreting graphs, evaluating claims, spotting misleading statistics—is essential for modern citizens.

Logical rules and modular arithmetic underpin coding systems. A Caesar cipher (logic-based substitution) and RSA encryption (based on number-theoretic difficulty) both rely on mathematical logic. Cryptography is logic and number theory applied to protect information.