Question

Find the sum of first 17 terms of an A.P whose 4th & 9th terms are -15 & -30 respectively.

Answer 1

T4= -15

a+3d= -15

a= -15-3d

T9= -30

a+8d= -30

-15-3d+8d= -30

5d= -30+15

5d= -15

d= -3

a= -15+(-3)(-3) => a= -15+9 = -6

T17=a+16d

= -6+16 (-3)

= -6-48= -52

n

Sn= ---(a+l)

2

17. 17.

S17= ---- (-6-52) = ------ × -58 = 17× -29

2. 2

= -493