Chapter 9: Sequences and Series

Q1. Find the sum of first 20 terms of the AP: 3, 7, 11, 15, ...

Solution

a = 3, d = 4, n = 20
S₂₀ = 20/2[2(3) + 19(4)] = 10[6 + 76] = 820

Q2. Find the sum of GP: 2, 6, 18, 54, ... to 6 terms.

Solution

a = 2, r = 3, n = 6
S₆ = 2(3² - 1)/(3 - 1) = 2(728)/2 = 728

Q3. Find the AM between 5 and 15.

Solution

AM = (5 + 15)/2 = 10

Q4. Find the 10th term of GP: 3, 6, 12, 24, ...

Solution

a = 3, r = 2
T₁₀ = 3 × 2² = 3 × 512 = 1536