20 Most Important Formulas for CBSE Class 10 Board Exam

A small set of formulas comes up in CBSE Class 10 board papers again and again. These are the 20 we would learn first, because so many numericals are built on them.

Physics (7 formulas)

1. Ohm's Law — V = IR. Almost every year. Direct 3-mark numerical.
2. Power in a circuit — P = VI = I^2 R = V^2/R. Both direct and combined-circuit problems.
3. Heating effect — H = I^2 R t (Joule's law). Common in 3-mark numerical.
4. Mirror formula — 1/f = 1/v + 1/u with sign convention. Concave/convex mirror problems.
5. Lens formula — 1/f = 1/v - 1/u. Convex lens image formation.
6. Magnification (mirrors) — m = -v/u. Always paired with mirror formula.
7. Magnification (lenses) — m = v/u (note: no negative sign, opposite convention). Trap!

Chemistry (6 formulas)

1. Combination — A + B \to AB. Identify by pattern.
2. Decomposition — AB \to A + B.
3. Displacement — A + BC \to AC + B (uses reactivity series).
4. Double displacement — AB + CD \to AD + CB.
5. pH definition — pH = -\log[H^+]. Buffer/salt questions.
6. Combustion — \text{Fuel} + O_2 \to CO_2 + H_2O. Balance carefully.

Maths (7 formulas)

1. Quadratic formula — x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}. Direct application.
2. Discriminant — D = b^2 - 4ac. Nature of roots question.
3. Sum & product of roots — \alpha + \beta = -b/a, \alpha\beta = c/a.
4. AP nth term — a_n = a + (n-1)d.
5. AP sum — S_n = \frac{n}{2}[2a + (n-1)d].
6. Section formula — \left(\frac{mx_2 + nx_1}{m+n}, \frac{my_2 + ny_1}{m+n}\right).
7. Trig ratios — \sin, \cos, \tan for standard angles 0°, 30°, 45°, 60°, 90°.

How to use this list

• Week 1: memorise all 20 formulas + one worked example each.
• Week 2: attempt one PYQ (previous year question) per formula.
• Week 3: solve mixed problems that use 2–3 formulas together.

Every one of the above has a detailed page on Board Formulas with derivation, worked example, common mistakes and practice questions. Bookmark the ones you struggle with.

Where these formulas earn marks

They show up in three kinds of questions:

• Direct application: put the numbers into one formula.
• Multi-step numericals: combine two or three formulas, such as resistors in a circuit followed by power.
• Definitions and derivations: state the formula, explain each symbol, or show where it comes from.

Practise all three with each formula, not only the first.