Question

if the n term of an A.P. is (2n+1) find the sum of first n term of the A. P.

Answer 1

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Answer 2

an=l=a+(n-1)d
2n+1=a+(n-1)d
a=2n+1-nd+d
a=n(2-d)+1+d.......(1)
sn=n/2(a+l)
=n/2{n(2-d)+1+d+2n+1}