Question

5,10,15 is an arithmetic progression find the sum of first 21 digits
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Answer 1

The sum of the first 21 terms of the AP is 1155.

Step-by-step explanation:

The sum of first n terms of an AP is:

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

Here,

a = 5

d = 5

n = 21

Compute the sum of first 21 terms of an AP as follows:

$S_{21}=\frac{21}{2}[(2\times 5)+(21-1)(5)]$

     $=\frac{21}{2}\times [10+(20\times 5)]$

     $=\frac{21}{2}\times 110$

     $=1155$

Thus, the sum of the first 21 terms of the AP is 1155.

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