Question

2. Calculate the sum of the first 20 terms of an arithmetic sequence with first

term 18 and common difference 11.

Answer 1

The sum of the first 20 terms is 2450

Explanation:

Sum of nth terms of an arithmetic sequence is

Sn = n/2 [2a+ (n -1)d]

Therefore, the sum of the first 20 terms

where first term = a = 18 and common difference = d = 11 is

S_20 = 20/2 [2(18) + (20 -1)11]

S_20 = 10 [36 + (19)11]

S_20 = 10 [36 + 209]

S_20 = 10 [245]

S_20 = 2450