I wonder if someone can help me. In chapter 15 of the prescribed book excercise 15.16 and excercise 15.17 uses [15_12] in their solutions. I can follow the solutions that are provided in the textbook up until these points, i do not understand how the author uses [15_12] going from one step to the next. Please see the solutions in the book to get an idea of what i'm referring to.
Can anyone show me how the author uses this line (15_12) to get to the next step in his solution...i just can't seem to grasp it. (maybe by showing me additional steps involved)