The sum of the first 10 terms of the geometric sequence -4, 6, -9,..., to the nearest tenth, is;
The answer is:
For the geometric series a=-4, r=-1.5 or -3/2, n=10.
Sn= a(r^n - 1)/r - 1
S10= -4((-1.5)^10 - 1)/-1.5 - 1
S10= 90.7
My question is from where they got r=-1.5 or -3/2 Please can somebody tell me why!!!
6/-4 = -1.5 = r
-9/6 = -1.5 = r
r is the ratio of every term to the one before it.