Question

counting by 7, Find the sum of first five number obtain ​

Answer 1

First five numbers = 7,8,9,10 & 11.

You can directly calculate the sum i.e

7 + 8 + 9 + 10 + 11 = 45

Or you can use formula of AP for which:-

a = 7. d = 1. n = 5

$sn = \frac{n}{2} (2a \: + (n - 1)d) \\ \\ = \frac{5}{2} (2 \times 7 + (5 - 1)1) \\ \\ = \frac{5}{2} (14 + 4) \\ \\ = \frac{5}{2} \times 18 \\ \\ = 5 \times 9 \\ \\ = 45$

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