While adding the first n terms of an arithmetic progression, a student pairs the first term with the last term, the second term with the second–last term, and so on.
Why does every such pair have the same sum?
(a) Because a+l equals the common difference of the progression.
(b) Because terms equally distant from the ends always add up to the constant value a+l.
(c) Because terms next to each other always add up to the same value since the common difference is constant.
(d) Because a+l is twice the common difference of the progression.