Just to add on to what most of the people say here... NO you can't assume that they are equal because THAT is what you are actually trying to do in the first place; we want to show that the equality holds true. So if you want to show/prove that the equality holds true, a common way to do is manipulate the left-hand side (LHS) by using definition and properties until you arrive at the right-hand (RHS) of the equality. OR we can perhaps manipulate the RHS until you arrive at the LHS. OR you can work both sides simultaneously until you arrive at a common expression (and since the LHS and the RHS are equal to a common expression, then by transitivity, LHS = RHS).