If you differentiate twice you get
You just differentiate each term in the sum above and divide by 2. After a few terms you should see a pattern then just turn it into an infinite sum.
etc...
So differentiate both sides...
etc...
differentiate again...
etc...
This is a series of the form...
.
How did I get this? Just differentiate twice. You don't even need to all that stuff I did above. Just it's good to see it done so you know what's going on.
Differentiating twice gives you . I.e.
So summarizing...
=>