Should I memorise all identities?

Yes. The four algebraic identities must be memorised exactly, including the middle term for (a±b)². They are used in expansion, factorisation, and numerical calculations.

Tamil Nadu Class 8 Maths Formula and Concept Bank — Samacheer Kalvi

Chapter 1 — Rational Numbers

Concept	Statement / Formula
Rational number	p/q where p, q ∈ ℤ and q ≠ 0
Standard form	q > 0 and HCF(p, q) = 1
Additive inverse	−p/q
Multiplicative inverse	q/p
Density property	Infinitely many rationals between any two rationals
Addition (unlike fractions)	Convert to LCM denominator, then add numerators
Division	Multiply by reciprocal of divisor

Chapter 2 — Measurements

Shape	Formula
Area of trapezium	½ × (a + b) × h
Area of rhombus	½ × d₁ × d₂
Perimeter of rhombus	4s
LSA of cube	4a²
TSA of cube	6a²
Volume of cube	a³
LSA of cuboid	2h(l + b)
TSA of cuboid	2(lb + bh + lh)
Volume of cuboid	lbh
Unit conversion	1 m³ = 1000 litres

Chapter 3 — Algebra (Identities)

Identity	Expansion
(a + b)²	a² + 2ab + b²
(a − b)²	a² − 2ab + b²
(a + b)(a − b)	a² − b²
(x + a)(x + b)	x² + (a + b)x + ab

Factorisation reverse:

a² + 2ab + b² = (a + b)²
a² − 2ab + b² = (a − b)²
a² − b² = (a + b)(a − b)

Chapter 4 — Geometry

Property / Rule	Statement
Angle sum of triangle	= 180°
Exterior angle theorem	Exterior angle = sum of two remote interior angles
SSS	All three sides equal
SAS	Two sides and included angle equal
ASA	Two angles and included side equal
RHS	Right angle, hypotenuse, one side equal
Parallelogram — sides	Opposite sides equal and parallel
Parallelogram — angles	Opposite angles equal; consecutive supplementary
Parallelogram — diagonals	Bisect each other
Rhombus — diagonals	Bisect at right angles; bisect vertex angles
Rectangle — diagonals	Equal in length

Chapter 5 — Coordinate Geometry

Concept	Formula / Fact
Origin	(0, 0)
Quadrant signs	I:(+,+) II:(−,+) III:(−,−) IV:(+,−)
Distance formula	√[(x₂−x₁)² + (y₂−y₁)²]
Distance from origin	√(x² + y²)

Chapter 6 — Statistics

Measure	Formula
Mean	Sum of all observations ÷ number of observations
Median	Middle value when data is arranged in order
Mode	Most frequently occurring value

Chapter 7 — Graphs

Concept	Statement
Linear graph	Straight line; represents constant rate of change
Slope (informally)	Steeper line = faster rate of change
Intersection	Point where two graphs meet; both equations satisfied

Quick Recall Test

Cover the right column and write each formula from memory. Check your answers and circle anything you missed. Revise circled formulae daily until the exam.

Frequently Asked Questions

Cover the formula column and try to recall each formula from the chapter and topic names alone. Any formula you cannot recall goes on your revision card.