# SPM 2021 Add Maths Trial Paper (Selangor) – Paper 1 (Question 5 & 6)

Question 5:
(a) Given a geometric progression with terms a, ar, ar2, ar3, … , arn-2, arn-1 and the sum of first n terms is Sn
Derive the formula for the sum of the first n terms, Sn for the geometric progression when |r| < 1. [3 marks]

(b) Hence, find the value of n for which the sum of the first 2n terms is 127/128 times of the sum of first n terms of a geometric progression which has a common ratio of -½. [4 marks]

Solution:
(a)

(b)