# Math Help - simple solution?

1. ## simple solution?

i have a simple question.
can someone please tell me how to get there?
I just dont get it.

2. Also,
What can i do in this case?
c(j)c(j + 2)???

3. Originally Posted by krispiekream
i have a simple question.
can someone please tell me how to get there?
I just dont get it.

$\sum_{j= 0}^\infty \left(0.9\right)^{2j}= \sum_{j=0}^\infty \left(0.9^2\right)^j= \sum_{j= 0}^\infty \left(0.81\right)^j$
is a geometric series with a= 1 and r= 0.81. Use the formula for the sum of a geometric series: $\sum_{j=0}^\infty ar^j= \frac{a}{1- r}$

4. thank you..i needed that..

5. I understand the first step now..