Question

Find the sum of first 17terms of an AP whose 4th and 9th term are - 15and -30 respectively

Answer 1

Answer:

-510

Step-by-step explanation:

Let a & d be the first term and common difference of the A.P.

4th term = a + 3d = -15

9th term = a + 8d = -30

Solving above two equations we have

a = -6, d = -3

Than the sum of first 17 terms is given by,

S = n/2 { 2a + (n -1)d} = 17/2 { 2 * (-6) + (17-1)(-3) }

S = -510