Question

#192 помогите........

Answer 1

a7=a1+6d=-100
{a9=a1+8d=-78
отнимем
-2d=-22
d=11
a1+66=-100
a1=-166
a15=a1+14d=-166+154=-12
S15=(a1+a15)*15/2=(-166-12)*15/2=(-78)*15=-1170