Question

You are told that an is an arithmetic sequence with a3=11 and a6=17. What is a15?

Answer 1

a15 = 35

Explanation:

Given:

a3 = 11

a6 = 17

Find:

a15 = ?

Computation:

a3 = 11 ⇒ a+ 2d = 11..........Eq1

a6 = 17 ⇒ a + 5d = 17...............Eq2

From Eq2 - Eq1

⇒ 3d = 6

⇒ d = 2

And a = 7

So,

⇒ a15 = a + 14d

⇒ a15 = 7 + 14 (2)

⇒ a15 = 7 + 28

⇒ a15 = 35