How do you multiply #(x + -1y)(x + y)#?
It is the case that
We also have
The shortcut is to know that if you have the form
That is the condition in this question so
Multiply everything inside the right hand side bracket by everything inside the left hand side bracket.
Notice that the minus follow the
So we have:
Each term in the 2nd bracket must be multiplied by each term in the 1st bracket.
now distribute the brackets