periodic functions

[scariari]

I'm doing a practice exam for a math test on thursday, wondering if anyone could help figure out how to get from one step to the next. i dont think that the background info is necessary for these two steps.

the file is attached (Adobe acrobat).

what i am wondering about is the answer under (2b).
If the solution for finding (del)u(x,y) were lettered a through d (4 steps, i am wondering how the professer got from b to c.

of course i have emailed him as well.

[nbo10]

Look in the last line of the 2b) paragraph.

\Delta \frac{16}{\pi^4 kl(k^2+l^2)}sin(k \pi x)sin(l \pi y) =
\frac{16}{\pi^4 kl(k^2+l^2)}(-\pi^2 (k^2 + l^2)sin(k\pi x)sin(l\pi y))



JMD