Find the sum of first 15 multiples of 8.

We need to find 8 + 16 + 24 + ….. + (15th term)

The above series forms an A.P. with a = 8 and d = 8.

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

⇒ S₁₅ = $\dfrac{15}{2}[2 \times 8 + (15 - 1)8]$
⇒ 2S₁₅ = 15[16 + 14(8)]
⇒ 2S₁₅ = 15[128]
⇒ 2S₁₅ = 1920
⇒ S₁₅ = 960

Hence, the sum of first 15 terms is 960.