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Find the sum of first seven numbers which are multiples of 2 as well as of 9.

Answers (1)

$\text{LCM of 2 and 9 is 18.} \\ \text{So the terms are } 18, 36, \ldots, 126 \\ \text{First term }(a) = 18$

$\text{Common difference }(d) = 36 - 18 = 18$

${{S}_{n}}=\frac{n}{2}\left[ 2a+\left( n-1 \right)d \right]$

$\text{Put } n = 7$

$\\ =\frac{7}{2}\left[ 36+108 \right] \\ =\frac{7}{2}\left[ 144 \right] \\$

$S_{7} = 7 \times 72 \\ S_{7} = 504$

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