The sum of the first 21 terms of the progressions,-18, -15, -12...is:(a)441 (b)336 (c)270 (d)252

Answer 1

This is arithmetic progression with first term a = - 18 and common difference d = 3

In AP sum of first n members is:

Sn = n • [ 2 a + ( n - 1 ) • d. ] / 2

In this case:

a = - 18

d = 3

n = 21

S21 =21 • [ 2 • ( - 18 ) + 20 • 3 ] / 2 =

21 • ( - 36 + 60 ) / 2 = 21 • 24 / 2 =

504 / 2 = 252