# LaGrange Error and power series

There's a homework problem that I've been struggling over:

Find a formula for the truncation error if we use 1 + x^2 + x^4 +x^6 to approximate 1/(1-x^2) over the interval (-1, 1).

Now, I assume that you need to use LaGrange error but I'm not sure how to proceed. Any help would be greatly appreciated.

HallsofIvy
$$E\le \frac{M}{(n+1)!}|x-a|^{n+1}$$