Question

evaluate 13 + 33 + 53 + 73 + ...... upto n terms.

Answer 1

a=13
d=20
nth term=13+(n-1)20
  =-7+20n
Sn=n/2(a+l)
=n/2(13+(-7+20n))
=n/2(6+20n)
=6n+20n^2/2
=3n+10n^2