Question

Find the sum of the first 15 multiples of 8.

Answer 1

Hi there!

The positive integers which are multiple of 8 are:

8, 16, 24... to 15 terms

It forms an arithmetic series. 

Now,

First term, a = 8

Common difference, d = 8

Number of terms, n = 15

Now, sum of first 15 terms of AP = (n/2) × {2a + (n-1)d}

                                               = (15/2) × {2 × 8 + (15-1)8}

                                               = (15/2) × (16 + 14 × 8)

                                               = (15/2) × (16 + 112)

                                               = (15 × 128)/2

                                               = 15 × 64                  

                                               = 960

Cheers!

Answer 2

Hi there!!
Refer the above picture..
I hope that will help you!! :)