Find the sum of first fifteen multiples of 8.

8, 16, 24,……,(upto 15th term).

The above is an A.P. with common difference equal to 8.

We know that,

Sum of n terms of an A.P. is given by,

∴ S_n = $\dfrac{n}{2}$ [2a + (n - 1)d]

Given,

a = 8

d = 16 - 8 = 8

n = 15

S₁₅ = $\dfrac{15}{2}$ [2(8) + (15 - 1)8]

= $\dfrac{15}{2} \times$ [16 + (14)8]

= $\dfrac{15}{2} \times$ [16 + 112]

= $\dfrac{15}{2} \times$ (128)

= 15 × 64

= 960.

Hence, S₁₅ = 960.