# Thread: Geometric sums and series.

1. ## Geometric sums and series.

Find the sum accurate to 1 decimal place, of the 20 terms of the geometric series having T1=15, and T2=20 answer is 14145.2

I do r= 20/15
a= 15
n=20

so 15((20/15)^20-1) over (20/15)-1

and I get 14187?

I dont totally understand the question, what is it Im doing wrong?

2. Hi

You are doing nothing wrong except the calculus itself

$\displaystyle 15\cdot \frac{\left(\frac{20}{15}\right)^{20}-1}{\frac{20}{15}-1} = 14145.2$

3. I got it thanks..