a1=7 a8 = -7. Найдите S20—?

Question

a1=7 a8 = -7. Найдите S20—?

Answer 1

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.

Answer 2

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