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

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

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Numbers divisible by 7 from 200 to 400 are 203,210,.............399

This sequence is an A.P.

Here , first term =a =203

common difference = 7.

We know , a_n = a+(n-1)d

              399 = 203+(n-1)7

      \Rightarrow \, \, 196 = (n-1)7

    \Rightarrow \, \, 28 = (n-1)

 \Rightarrow \, \, n=28+1=29

S_n = \frac{n}{2}[2a+(n-1)d]

       = \frac{29}{2}[2(203)+(29-1)7]

     = \frac{29}{2}[2(203)+28(7)]

      = 29\times 301

     = 8729

The  sum of numbers divisible by 7from 200 to 400 is 8729.

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