arithmetic progression
posted 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 ?

use the first two facts to find d,a1
Sn=n/2(2a1+(n1)d)
you will have two unknowns, two equations.
then, find s34