T(n)= T(n-1) +n
T(n-1)= T(n-2) +(n-1) [putting n=n-1]
T(n-2)= T(n-3) +(n-2) [again putting n=n-1]
............
T(1)= T(0) +1
----------------------------------------------------
T(n)= T(0) +n+(n-1)+............+1 [Adding all the terms]
T(n)=2+n(n+1)/2
=2+(n2+n)/2
T(n)=1/2(n2+n+4)