Question

find sn, fo the given arithmetic series a1-10, d=-9, n=15

Answer 1

is that a1 = 10 or -10?

Answer 2

Well, anyway... I'm going to assume its -10
The sum of an arithmetic series:
n
Sn = -- (A1 + An)
2

An = A1 + d (n-1)
= -10 + -9(n-1)
= -10 -9n +9
= -9n - 1
So A15 = -9(15) -1 = -136
15
Sn = --- (-10 + -136) = -1095
2

Answer 3

@christiner14