Question

Q. The sum of the first two terms of a GP is -3 and the sum of the first 4 terms is -15. Find the sequence.

• A. 3, -6, 12, -24
• B. 6, -12, 24, - 48
• C. 1, 2, 4, 6
• D. 3, -15, 75, -375
• E. None of these

Answer 1

The correct option is A 3, -6, 12, -24

First term = a
Common ratio = r
a +ar = -3
$a\times (1+r+r^2+r^3)=-15=a(1+r)+a{r}^2(1+r)=-15=a(1+r)(1+r^2)=-15=1+r^2=5r=\pm 2a=-1forr=2,a=3forr=-2$
So possible terms are
-1, -2, -4, -8,
Or 3, -6, 12, -24