Question

Q. find Sn for AP :- 7,9,11,...,213
find answer

Answer 1

First term, a = 7

Last term, a_n = 213

Common difference, d = 9-7 =2

$a_n = a + (n - 1)d \\ 213 = 7 + (n - 1)2 \\ 213 - 7 = 2n - 2 \\ 206 + 2 = 2n \\ n = \frac{208}{2} = 104$

Sum of an AP

$S_n = \frac{n}{2}(a + a_n) \\ = \frac{104}{2} (7 + 213) \\ = 52 \times 220 \\ = 11440$

Answer 2

You can use Formula of Sn = n/2 {2a+(n-1)d}