Posted by **Bishnu** on Tuesday, January 8, 2013 at 9:28am.

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 ?

- arithmetic progression -
**bobpursley**, Tuesday, January 8, 2013 at 9:35am
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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