Thread: Another Geometric Series Question.

A geometric series has first term a and common ratio r. The sum of the first two terms of the geometric series is 7.2. The sum to infinity of the series is 20. Given that r is positive, find the values of r and a. [6]

First I look at the sum to infinity and see that 20 - 20r = a

And then I put 7.2 =
And subbing what I worked out above,

And then I end up with 7.2 =

At this point I don't know what I'm doing now. Could someone please help?

I think you're on the right track, but you maybe went slightly askew....

From the formula for the sum of a geometric series, you have arrived at , where is the first term of the series.

Now use the fact that the second term is :

. . . . .

Solve by square roots.