I've made this exercise below. My broblem is that my solution doesn't match the solution in the book.Where did I make the mistake?
This is my result.
Below it's the correct result:
You're almost right.
The mistake is here :
so you should have been warned there's a problem here =)
Don't forget the formula, which should give twice more solutions.
Now, you can shorten it.
In fact, you can notice that
Try to gather all of these, you'll get that :