Question

if a=4, d=(-5), n=15, then find S15​

Answer 1

Answer:

S15 = n/2[2a+(n-1)]

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

S15 = 15/2(-72)

S15= 15×-36

S15= -540

Answer 2

S15 =

$\frac{15}{2} {2a + (n - 1)d}$

= 15/2 ×{8-70}

15/2 × -62

=-465

hope it helps you