Question

Find the sum of first fifteen multiples of 8.

Answer 1

First fifteen multiples of 8 are 8, 16, 24, ..., 120.

Here $$a = 8, \ d = 8, \ n = 15$$. 

$$S_{15} = \frac{15}{2}[2(8) + 14(8)]$$

$$= \frac{15}{2}(16 + 112)$$

$$= 15(64)$$

$$= 960$$