The sum up to n terms of an AP in which the first term is a, last term is l, and the common difference is d, is ________
We know that sum upto n terms of an AP with first term a and common difference d is
We know that a +(n-1)d is the nth term, which is the last term.
Substituting the value of last term i.e. a +(n-1)d equals to l in formulae (1).