Question

The sum of first five multiples of 3 is ______.

Options

• 45

• 55

• 65

• 75

Answer 1

The sum of first five multiples of 3 is 45.

Explanation:

The first five multiples of 3 are 3, 6, 9, 12 and 15.

Here, first term, a = 3

Common difference,

d = 6 - 3 = 3

And number of terms, n = 5

∴ `S_5 = 5/2 [2a + (5 - 1)d]`  .....`[because S_n = n/2 {2a + (n - 1)d}]`

= `5/2 [2 xx 3 + 4 xx 3]`

= `5/2 (6 + 12)`

= `5 xx 9`

= 45