Question

Find the sum of first 15 terms of sequences having nth term as:

y_n = 9 – 5n

Answer 1

Given an A.P. whose n^th term is given by y_n = 9 – 5n

To find the sum of the n terms of the given A.P., using the formula,

S_n = \(\frac{n(a \,+\, l)}{2}\)

Where, a = the first term l = the last term.

Putting n = 1 in the given y_n, we get

a = 9 – 5(1) = 9 – 5 = 4

For the last term (l), here n = 15

a₁₅ = 9 – 5(15) = -66

So, S_n = \(\frac{15(4 \,–\, 66)}{2}\)

= 15 x (-31)

= -465

Therefore, the sum of the 15 terms of the given A.P. is S₁₅ = -465