Question

finf the sum of all the natural numbers between 10 and 200 which are divisible by 7.

Answer 1

first number after 10 divisible by 7 = 14 = $t_{1}$ of the AP

last number before 200 divisible by 7 = 196 = 14 + 26*7 = $t_{27}$ of the AP

let the required sum be 'S'

S is a summation of an arithmetic progression

S = (27/2)(2*14 + 26*7)

S= 2835

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