Asked by Bishnu

If sum of first 21 terms of an arithmetic progression is 13 and that of first 13 terms is 21, show that sum of first 34 terms is -34 ?
Answered by bobpursley
use the first two facts to find d,a1

Sn=n/2(2a1+(n-1)d)

you will have two unknowns, two equations.

then, find s34
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