I'm really not sure where it's appropriate to post this, but I was having difficulty with this problem. Since there's quite a bit of background, the link to the equation is:

http://www.physics.byu.edu/faculty/p...ysics571L6.pdf

The proof of equation 6.8 is what's troubling me. I've wrestled with it for about 4 hours now and have gotten nowhere.

I'd like to add that this isn't my assignment; I'm writing a paper and this .pdf came up when I was researching background information. It would just be nice to include the proof in the introduction.

I'm assuming the final form of the equation should have the two sides equivalent other than an offset of +/- 1/f or f.

Thanks.