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Find the sum of all the numbers between $200$ and $400$ which are divisible by $7$.

$\begin{array}{1 1}9030 \\ 8127 \\ 8428 \\ 8729 \end{array} $

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  • $n^{th}$ term of an A.P.$=t_n=a+(n-1)d$
  • sum of $n$ terms of an A.P.$=\large\frac{n}{2}$$(l+a)$ where $l=t_n$
Numbers between $200$ and $400$ divisible by $7$ forms the sequence
This sequence is an A. P. with first term $a=203$ and
common difference $d=7$
The last term or $n^{th}$ term of this sequence is $t_n=399$
We know that $n^{th}$ term of an A.P. = $a+(n-1)d$
or $n=29$
$\Rightarrow\:$The sum of the required numbers =
answered Mar 28, 2014 by rvidyagovindarajan_1

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