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

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

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Natural numbers beteeen 200 and 400 which are divisible by 7 are 203,210,217,........,399.These numbers form an A.P. with a=203,d=7 and l=399.
Let there be n terms in this A.P. with a=203, d=7 and l=399.
399=203+(n-1) imes7
Rightarrow 7(n-1)=196Rightarrow n-1=28Rightarrow n=29
Therefore the Sum=fracn2(a+l)=frac292(203+399)
=frac292(602)=29 imes301=8729
Hence the required sum=8729

Posted by

Deependra Verma

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