Midsem Maths Cheatsheet
Math-I mid-sem · Zenith School of AI

Formula Cheatsheet

Every topic on the syllabus on one page. Highlighted items are the ones the mock test actually used. Part A is 40 one-mark MCQs; Part B is 4 of 5 fifteen-mark written questions (in the mock: 3 logic, 1 area between curves, 1 combinations + probability).

Sets priority

  • Roster {1,2,3} · builder {x ∈ Z | −2 ≤ x < 3} = {−2,−1,0,1,2}
  • A ∪ B in either · A ∩ B in both · A − B in A not B = A ∩ B′ · A′ = U − A
  • Symmetric difference A ⊕ B = (A−B) ∪ (B−A)
  • De Morgan: (A∩B)′ = A′∪B′, (A∪B)′ = A′∩B′
  • |A∪B| = |A|+|B|−|A∩B|
  • |A∪B∪C| = ΣA − Σ(pairs) + |A∩B∩C|
  • Neither = |U| − |A∪B|
  • Subsets of n-set: 2ⁿ; proper subsets 2ⁿ − 1; power set size 2ⁿ
  • |A×B| = |A|·|B|, pairs are ordered (a,b)
  • Disjoint: A ∩ B = ∅. ∅ is a subset of every set.

Probability priority

  • P(E) = favourable / total, 0 ≤ P ≤ 1
  • P(A′) = 1 − P(A) · "at least one" = 1 − P(none)
  • P(A∪B) = P(A)+P(B)−P(A∩B)
  • P(A−B) = P(A) − P(A∩B)
  • P(A|B) = P(A∩B) / P(B)
  • Independent ⇔ P(A∩B) = P(A)·P(B)
  • Mutually exclusive ⇔ P(A∩B) = 0
  • 3 events: P(A∪B∪C) = ΣP − ΣP(pairs) + P(A∩B∩C)
  • Two dice: 36 outcomes. 3 coins: 8 outcomes.

Matrices priority

  • Order m×n = rows × columns. Add/subtract only same order.
  • Product (m×n)(n×p) = m×p: entry = row of A · column of B
  • AB ≠ BA in general · (AB)ᵀ = BᵀAᵀ · AI = IA = A
  • Transpose: rows become columns. Symmetric: Aᵀ = A; skew: Aᵀ = −A
  • 2×2 inverse: [a b; c d]⁻¹ = (1/(ad−bc))·[d −b; −c a]
  • General: A⁻¹ = adj(A)/|A|, adj = transpose of cofactor matrix
  • Singular ⇔ |A| = 0 ⇔ no inverse
  • System AX = B ⇒ X = A⁻¹B

Determinants priority

  • |a b; c d| = ad − bc
  • 3×3: expand along row 1 with signs + − +: a(ei−fh) − b(di−fg) + c(dh−eg)
  • Cofactor Cᵢⱼ = (−1)^(i+j)·Mᵢⱼ (Mᵢⱼ = minor, delete row i col j)
  • |AB| = |A||B| · |Aᵀ| = |A| · |kA| = kⁿ|A| (n×n) · |A⁻¹| = 1/|A|
  • Swap two rows: sign flips. Two equal rows or a zero row: det = 0
  • Triangular matrix: det = product of diagonal
  • Cramer: x = Dₓ/D, y = D_y/D (replace that column with constants)
  • Area of triangle = ½|x₁(y₂−y₃)+x₂(y₃−y₁)+x₃(y₁−y₂)|

Vectors

  • |v| = √(x²+y²+z²) · unit vector v/|v|
  • Dot: a·b = a₁b₁+a₂b₂+a₃b₃ = |a||b|cos θ
  • Perpendicular ⇔ a·b = 0 · parallel ⇔ a = k·b
  • Cross (3D): a×b = det of [i j k; a₁ a₂ a₃; b₁ b₂ b₃], magnitude = area of parallelogram

Propositional logic Part B

  • Precedence: ( ) → ¬ → ∧ → ∨ → ⇒, ⇔ (⇔ evaluated last)
  • P⇒Q is false only when T ⇒ F. False P ⇒ anything is T (vacuous)
  • P⇔Q true when same · P⊕Q true when different
  • P⇒Q ≡ ¬P ∨ Q · ¬(P⇒Q) ≡ P ∧ ¬Q
  • Converse Q⇒P · Inverse ¬P⇒¬Q · Contrapositive ¬Q⇒¬P (≡ original)
  • Truth table rows = 2ⁿ (n distinct variables)
  • Tautology always T (P∨¬P) · Contradiction always F (P∧¬P) · Contingency mixed
  • De Morgan: ¬(P∧Q) ≡ ¬P∨¬Q, ¬(P∨Q) ≡ ¬P∧¬Q
  • Identity P∧T≡P, P∨F≡P · Domination P∨T≡T, P∧F≡F
  • Absorption P∨(P∧Q)≡P, P∧(P∨Q)≡P · Distributive P∧(Q∨R)≡(P∧Q)∨(P∧R)

Rules of inference Part B

RuleFromConclude
Modus PonensP⇒Q, PQ
Modus TollensP⇒Q, ¬Q¬P
Hypothetical SyllogismP⇒Q, Q⇒RP⇒R
Disjunctive SyllogismP∨Q, ¬PQ
ResolutionP∨Q, ¬P∨RQ∨R
Addition / SimplificationP / P∧QP∨Q / P
ConjunctionP, QP∧Q

Fallacies: affirming the consequent (P⇒Q, Q ∴ P ✗), denying the antecedent (P⇒Q, ¬P ∴ ¬Q ✗). Contradiction proof: assume ¬conclusion, derive X ∧ ¬X.

Predicate logic Part B

  • ∀ pairs with ⇒, ∃ pairs with ∧
  • "All S are L": ∀x(S(x)⇒L(x)) · "Some S are L": ∃x(S(x)∧L(x))
  • "No S is H": ∀x(S(x)⇒¬H(x)) ≡ ¬∃x(S(x)∧H(x))
  • "Only P get D": ∀x(D(x)⇒P(x)) (the thing after "only" goes on the right)
  • Negate: flip every quantifier, negate the inside: ¬∃x∀y Q ≡ ∀x∃y ¬Q
  • ¬∀x(P⇒Q) ≡ ∃x(P ∧ ¬Q)
  • Universal Instantiation: from ∀x P(x) get P(c) for any named c
  • ∀x∃y ≠ ∃y∀x (order matters). ∃! = exactly one

Digital logic

  • AND ·, OR +, NOT A′. NAND/NOR are universal gates
  • K-map order 00, 01, 11, 10 (Gray code); edges wrap around
  • Group 1s in rectangles of 1, 2, 4, 8 (as large as possible, overlaps allowed)
  • In each group keep only variables that don't change
  • SOP from 1s; minterms m(i) use variable = 1 → plain, 0 → complemented

Counting Part B

  • AND ⇒ multiply · OR ⇒ add
  • ⁿPᵣ = n!/(n−r)! order matters · ⁿCᵣ = n!/(r!(n−r)!) order doesn't
  • ⁿCᵣ = ⁿCₙ₋ᵣ · 0! = 1 · ⁷C₃ = 35
  • Identical items: n!/(p!q!…)
  • Circle (n−1)! · necklace/bracelet (n−1)!/2
  • Together: tie as 1 block, then × internal arrangements
  • Apart (gap): arrange others, choose gaps (k+1)Pᵣ
  • Handshakes / matches = ⁿC₂; lines from n points ⁿC₂; triangles ⁿC₃; diagonals n(n−3)/2
  • "At least one" = total − none
  • Grid paths m right, n up: (m+n)Cₘ

Binomial theorem

  • (a+b)ⁿ = Σ ⁿCₖ aⁿ⁻ᵏ bᵏ, n+1 terms
  • General term Tₖ₊₁ = ⁿCₖ aⁿ⁻ᵏ bᵏ; for a coefficient, set power of x = target, solve k
  • Sum of coefficients: put every variable = 1 ⇒ 2ⁿ
  • Alternating sum ⁿC₀ − ⁿC₁ + … = 0
  • Pascal row 5: 1 5 10 10 5 1

AP and GP revision

  • AP aₙ = a + (n−1)d · Sₙ = n/2·[2a+(n−1)d] = n/2·(a+l)
  • n terms from a to l: n = (l−a)/d + 1 · AM = (a+b)/2 · 3 terms: a−d, a, a+d
  • GP aₙ = a·rⁿ⁻¹ · Sₙ = a(rⁿ−1)/(r−1)
  • Infinite GP S∞ = a/(1−r), needs |r| < 1
  • GM = √(ab) · AM ≥ GM · 3 terms: a/r, a, ar
  • Repeating decimal: 0.ababab… = ab/99; 0.363636… = 36/99 = 4/11
  • Bouncing ball from h, ratio r: distance = h + 2·hr/(1−r)

Patterns

  • 1st differences constant ⇒ linear; 2nd constant ⇒ quadratic; 3rd constant ⇒ cubic
  • n³ has quadratic 1st differences (7, 19, 37…)
  • Quadratic nth term: an²+bn+c, with 2a = 2nd difference
  • Triangular n(n+1)/2 · Pentagonal n(3n−1)/2 · Hexagonal n(2n−1)
  • Centered square n²+(n−1)² · squares in n×n grid n(n+1)(2n+1)/6
  • Rectangles in n×n grid [(n+1)C₂]²

Functions, logs, limits

  • Domain: inside √ ≥ 0, denominator ≠ 0, inside log > 0
  • Range = actual outputs; codomain = allowed outputs
  • Injective: f(a)=f(b) ⇒ a=b (horizontal line test). Surjective: range = codomain. Bijective: both, has inverse
  • Inverse: write y = f(x), swap x and y, solve for y
  • log(ab)=log a+log b, log(a/b)=log a−log b, log aⁿ = n log a, e^(ln x) = x
  • Limits: plug in first. 0/0 ⇒ factor and cancel. x→∞ ⇒ divide by highest power
  • lim (x²−9)/(x−3) at 3 = lim (x+3) = 6

Derivatives

  • d/dx xⁿ = n xⁿ⁻¹ · constant → 0 · d/dx (kx) = k
  • d/dx eˣ = eˣ · d/dx ln x = 1/x
  • Average rate = (f(b)−f(a))/(b−a) (secant slope)
  • Tangent at x=a: y − f(a) = f′(a)(x − a)

Integrals Part B

  • ∫xⁿ dx = xⁿ⁺¹/(n+1) + C (n ≠ −1) · ∫1/x dx = ln|x| + C · ∫eˣ dx = eˣ + C
  • FTC: ∫ₐᵇ f dx = F(b) − F(a) = net signed area
  • Linearity: ∫(3f+2g) = 3∫f + 2∫g · ∫ₐᵇ = −∫ᵇₐ · ∫ₐᶜ = ∫ₐᵇ + ∫ᵇᶜ
  • Area between curves = ∫ₐᵇ (top − bottom) dx, limits = intersection x's
  • u-sub: pick u = inside, du = u′ dx, change bounds for definite
  • ∫₀³ 2x dx = 9 · ∫₀^(ln2) eˣ dx = 1

Descriptive statistics

  • Mean μ = Σx/N · median = middle of sorted (avg of 2 middles if even) · mode = most frequent
  • Outliers pull the mean most; skewed data ⇒ use median. Right skew: mean > median
  • Range = max − min · σ² = Σ(x−μ)²/N = Σx²/N − μ²
  • σ = √variance, so variance = σ²
  • Add c to all: mean + c, σ unchanged
  • Multiply all by k: mean × k, σ × |k|, variance × k²
  • Empirical: 68% within ±1σ, 95% ±2σ, 99.7% ±3σ
  • Combined mean = (n₁m₁ + n₂m₂)/(n₁+n₂)

Mental math

  • ×5 = ×10 ÷ 2 · ×25 = ×100 ÷ 4 · ÷5 = ×2 ÷ 10
  • ×11: split and add, 45×11 → 4|9|5 = 495
  • N5² = N(N+1) then append 25: 75² = 5625
  • Near 100: 96×93 → 96−7 = 89 | 4×7 = 28 → 8928
  • Difference of squares: 75×45 = 60² − 15² = 3600 − 225 = 3375
  • 1/8 = .125, 1/6 = .1667, 1/7 = .142857…, k/9 = 0.kkk…, k/11 = 0.(9k)(9k)…
  • 3/4 + 2/5 = 0.75 + 0.40 = 1.15

Number theory

  • Divisible by 3/9: digit sum · 4: last two digits · 8: last three · 11: alternating sum ÷ 11
  • 7: drop last digit, subtract double of it, repeat
  • 6 = 2 and 3 · 36 = 4 and 9 (coprime parts)
  • Prime test: try primes up to √n
  • N = pᵃqᵇrᶜ ⇒ factors (a+1)(b+1)(c+1); odd count ⇔ perfect square
  • GCD = min powers · LCM = max powers · GCD×LCM = a×b
  • Trailing zeros of n! = ⌊n/5⌋ + ⌊n/25⌋ + …

Word problems

  • "4 more than 3 times x is 25" ⇒ 3x + 4 = 25
  • "5 less than x" ⇒ x − 5 (order flips)
  • Consecutive integers x, x+1; consecutive even 2x, 2x+2
  • Rate = amount ÷ time (5 blocks / 2 h = 2.5 per hour)
  • Profit = Revenue − Total cost; Revenue = price × quantity; break-even: R = C

Part B answer template

  • Define every variable in words first ("Let E: the user is external")
  • Write the rule you use at each line (e.g. "by Modus Tollens on 2, 3")
  • Number your proof lines; cite line numbers
  • Calculus: show F(x), then F(b) and F(a) separately, then subtract
  • Counting: write the formula, then the substituted numbers, then the value
  • Box the final answer of each part, because later parts reuse it