The first term, last term and common difference of an A.P are respectively a, l and 1. Prove that the sum of this A.P. is `(1)/(2)(a+b)(1-a+b).`

Explanation

To prove that the sum of the arithmetic progression (A.P.) with first term \( a \), last term \( l \), and common difference \( d = 1 \) is given by

\[ S = \frac{1}{2}(a + b)(1 - a + b), \]

we will follow these steps:

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