Given: an = 9 – 5n
Taking n = 1,
a1 = 9 – 5(1) = 9 – 5 = 4
Taking n = 2,
a2 = 9 – 5(2) = 9 – 10 = -1
Taking n = 3,
a3 = 9 – 5(3) = 9 – 15 = -6
Therefore the series is 4, -1, -6, …
So, a = 4, d = a2 – a1 = -1 – 4 = -5
Now, we have to find the sum of the first 15 terms of the AP
⇒ S15 = 15 × (-31)
⇒ S15 = -465
Hence, the sum of 15 terms is -465.