I want to find the sum of the series:1+3+6+10+15+...upto n terms.what has gone wrong in my code?

Question

What is wrong in my code
https://code.sololearn.com/cUhXowIU2QvM/?ref=app

Answer 1

Line 6:
a=1, d=3;

Lines 9-11:
for(i=3;i<=n+1;i++)
{a+=d;
d+=i;}