Question

Прогрессия.
Дано:
а3+а7=-12
Найти: S10

Answer 1

a3+a7=a1+2d+a1+6d=2a1+8d=-12⇒a1+4d=-6⇒a1=-6-4d
S10=(2a1+9d)*10/2=(-12-8d+9d)*5=-60+5d