CBSE · Class 6-12

Back to Arithmetic Progressions

Class 10 · Mathematics (Standard) · Unit 5: Arithmetic Progressions

Sum of First n Terms

Lesson 3 of 6

S_n = \dfrac{n}{2}\big(2a + (n-1)d\big)

. If the first term

a

and last term

l

are known,

S_n = \dfrac{n}{2}(a + l)

. To find how many terms sum to a target

T

, solve

S_n = T

(a quadratic in

n

).

General Term ($n$-th term)

Selecting Terms Symmetrically

All lessons in this unit