Question

The nth term of an A.P. is given by ( -4n + 15 ) . Find the sum of first 15 terms of this A.P.

Answer 1

Answer:

S15 = -150

Step-by-step explanation:

nth term = (-4n + 15)

1st term = -4(1)+15 = 11

2nd term = -4(2)+15 = 7

3rd term = -4(3)+15 = 3

a = 11 , d = 7-11 = -3

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

S15 = 15/2[2(11)+14(-3)]

= 15[11+(7×-3)]

= 15(11-21)

= 15×-10

= -150