Question

Find the sum of first five multiples of 3?

Answer 1

first 5 multiples are 3, 6,9 ......

a=3

d=3

n=5

Sn= n/2 [2a+(n-1)d]

= 5/2 [ 2×3+(5-1)3 ] = 45

so putting in all d values we get the answer to be 45.