A1=7 a8 = -7. Найдите S20—?

a(1)=7 a(8)=-7 S(20)-?   a(8)= a(1)+7d -7=7+7d 7d=-14 d=-2   S(20)=(2a(1)+19d) / 2 * 20 s(20)=(2*7+19*(-2))/2 * 20 = (14-38)/2*20=-240

1.a8=a1+7d 7d=a8-a1 7d=-7-7 7d=-14 d=-2 2.a20=a1+19d a20=7+19*(-2)=7-38=-31 S20=(a1+a20)/2*20= (7-31)/2*20=-24/2*20=-240.