Question

find the sum of first 17 term of an AP whose 4 and 9 term is -15 and -30 respectively​

Answer 1

Answer:

a+3d = –15....(i)

a+8d = –30....(ii)

apply (ii) – (i), we get

5d = –15

d = –3

putting value of d in (i),

a+3(–3) = –15

so, a = –15+9 = –6

$s_{17} = \frac{17}{2} (2 \times ( - 6) + 16 \times ( - 3))$

$= \frac{17}{2} ( - 12 - 48) = - 60 \times \frac{17}{2} =$

$- 30 \times 17 = - 510$

Ans.