Question

if a=4, d=(-5), n=15 then S15​

Answer 1

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

given :

a=4

d= -5

n=15

=) S15= 15/2 [2 (4)+ (15-1) -5]

=) S15= 15/2 [8 + (14)-5]

=) S15= 15/2 [ 8 -70]

=) S15= 15/2 * -62

=) S15= 15*-62/2

=) S15= 15*-31

=) S15= -465