Hi all, I am new to this forum but I think this may be the first post of many! Trying to do some revision before my new course starts this year, and have got stuck. This algebra stems from the use of the sum rule in a differentiation - in my answer I got as far as the first line in the solution, but can't see how they rearrange this to give the second line - it is the part in the red box that I am stuck on. How do they get to this?
I have created a jpg from the pdf file of the solutions booklet, hopefully people can read it . It's algebra I'm stuck on rather than the calculus part, so hopefully this is the correct forum for it.