Question

IN AN AP IF T18=15 THEN FIND S15

Answer 1

You are given

T4 = 15 = T1 + 3*d, (1)

T9 = 30 = T1 + 8*d. (2)

Distract (1) from (2) and get 5d = 15, d = 3.

Then from (1) find T1 = 15 - 3*d = 15 - 9 = 6.

So, your AP has the first term T1 = 6 and the common difference 3.

Now apply the formula for the sum of AP.

Answer 2

I think S15=3 is the answer