Answers

2016-03-16T21:10:44+05:30
A+13d =2(a+7d)

-a-d=0 ⇒a+d=0..........(1)

a+5d=-8 ........(2)

(2)- (1)

4d=-8⇒d=-2

Substitute and find a = 2

a20= 2+19*-2 = 2-38=-36

S20 = 10( 2+ (-36))= 10 (-34) = -340

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