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Tamil Nadu Class 9 Maths Formula and Concept Bank — Samacheer Kalvi

Complete formula bank for Class 9 Maths Samacheer Kalvi with all formulas, identities, theorems, and key concepts from every chapter.

Updated 11 May 20263 min read

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Chapter 1 — Set Language

FormulaExpression
Union countn(A ∪ B) = n(A) + n(B) − n(A ∩ B)
Three-set unionn(A ∪ B ∪ C) = n(A)+n(B)+n(C)−n(A∩B)−n(B∩C)−n(A∩C)+n(A∩B∩C)
Power setn(P(A)) = 2^n(A)
Proper subsets2^n(A) − 1
De Morgan's Law 1(A ∪ B)' = A' ∩ B'
De Morgan's Law 2(A ∩ B)' = A' ∪ B'

Chapter 2 — Real Numbers

FormulaExpression
Surd multiplication√a × √b = √(ab)
Surd division√a / √b = √(a/b)
Conjugate rationalisation(a+√b)(a−√b) = a² − b
Exponent productaᵐ × aⁿ = aᵐ⁺ⁿ
Exponent power(aᵐ)ⁿ = aᵐⁿ
Fractional exponenta^(1/n) = ⁿ√a
Negative exponenta⁻ⁿ = 1/aⁿ

Chapter 3 — Algebra

IdentityExpression
Cube expansion +(a+b)³ = a³ + 3a²b + 3ab² + b³
Cube expansion −(a−b)³ = a³ − 3a²b + 3ab² − b³
Sum of cubesa³+b³ = (a+b)(a²−ab+b²)
Difference of cubesa³−b³ = (a−b)(a²+ab+b²)
Three variable square(a+b+c)² = a²+b²+c²+2ab+2bc+2ca
Three variable cubesa³+b³+c³−3abc = (a+b+c)(a²+b²+c²−ab−bc−ca)
Remainder TheoremRemainder of p(x)÷(x−a) = p(a)
Factor Theorem(x−a) is factor ↔ p(a) = 0

Chapter 5 — Coordinate Geometry

FormulaExpression
Distance formulad = √[(x₂−x₁)² + (y₂−y₁)²]
MidpointM = ((x₁+x₂)/2, (y₁+y₂)/2)
Section (internal)P = ((mx₂+nx₁)/(m+n), (my₂+ny₁)/(m+n))
CentroidG = ((x₁+x₂+x₃)/3, (y₁+y₂+y₃)/3)

Chapter 6 — Trigonometry

IdentityExpression
Pythagorean 1sin²θ + cos²θ = 1
Pythagorean 21 + tan²θ = sec²θ
Pythagorean 31 + cot²θ = cosec²θ
Complementary sin/cossin(90°−θ) = cosθ; cos(90°−θ) = sinθ
Complementary tan/cottan(90°−θ) = cotθ; cot(90°−θ) = tanθ

Chapter 7 — Mensuration

ShapeLSATSAVolume
Cuboid2h(l+b)2(lb+bh+lh)lbh
Cube4a²6a²a³
Cylinder2πrh2πr(r+h)πr²h
Coneπrlπr(r+l)(1/3)πr²h
Sphere—4πr²(4/3)πr³
Hemisphere2πr²3πr²(2/3)πr³

Slant height: l = √(r² + h²)

Chapters 8–9 — Statistics and Probability

FormulaExpression
Mean (grouped)Σfx / Σf
Median (grouped)L + [(N/2−cf)/f] × h
Mode (grouped)L + [(f₁−f₀)/(2f₁−f₀−f₂)] × h
Empirical relationshipMode = 3 Median − 2 Mean
ProbabilityP(E) = n(E)/n(S)
ComplementP(E') = 1 − P(E)

Frequently Asked Questions

Print or copy this page into your notebook. Before each revision session, read through it. In the final week, test yourself by covering the right column and trying to recall each formula from memory.