A7=-100,a9=-78 a15=? S15=?

Question

A7=-100,a9=-78 a15=? S15=?

Answer 1

a8=(a7+a9)/2=(-100-78)/2=-89     d=a8-a7=-89+100=11

a7=a1+6d    a1=-100-6*11=-166

a15=a1+14d=-166+154=-12

S15=(a1+a15)/2 *15=1/2(-166-12)*15=-1335