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8. Sequences and Series
medium
Find the sum of all numbers between $200$ and $400$ which are divisible by $7.$
A
$8729$
B
$8729$
C
$8729$
D
$8729$
Solution
The numbers lying between $200$ and $400 ,$ which are divisible by $7,$ are $203,210,217 \ldots .399$
$\therefore$ First term, $a=203$
Last term, $I=399$
Common difference, $d=7$
Let the number of terms of the $A.P.$ be $n.$
$\therefore a_{n}=399=a+(n-1) d$
$\Rightarrow 399=203+(n-1) 7$
$\Rightarrow 7(n-1)=196$
$\Rightarrow n-1=28$
$\Rightarrow n=29$
$\therefore S_{29}=\frac{29}{2}(203+399)$
$=\frac{29}{2}(602)$
$=(29)(301)$
$=8729$
Thus, the required sum is $8729 .$
Standard 11
Mathematics
