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