Nth term of Arithmetic progression
Summation of Arithmetic progression
\( s =\frac{n}{2}(a_1 +a_2)\)
But: \(a_n = a_1 +d(n-1)\)
Thus:
\(S = \frac{n}{2} [a_1 +a_1 +d(n-1)]\)
\(S = \frac{n}{2} [2a_1 +d(n-1)]\)