Question

How many terms of the sequence -8,-6,-4 must be taken so that their sum is 70.

Answer 1

-8-6-4 = -18

-18 × -5 = 70

Ans -5

-ArjunIsCool

Answer 2

here AP is -8, -6, -4.....
their sum is 70
therefore
a= -8

d=-6+8= 2

sum=n/2(2a+(n-1)d

70=n/2(2X(-8)+(n-1)X 2

140=2n^2 -18n

0=2n^2- 18n -140

0=n^2 -9n -70

0=n^2-14n +5n -70

0=n(n-14)+5(n-14)

0=(n-14)(n+5)

n=-5 or 14

Both the value of n, being positive integers are valid. We get double answer because sum of 5th to 14th term is zero,as some terms are positive and some are negative.