My actual question doesn't have to do with what's said in the title, I'm having trouble with the derivative portion.
This is from the solution:
I am fine from the point I have to take the derivative of "r" and all the plugging in that goes on, but the step right after that is what I'm confused about. I can't figure out how they came up with the final step you see above in the picture. What simplifying was done to obtain that part?