You don't actually need that result. It's just more obvious if you have it. It's not necessarily part of your problem, so I wouldn't worry too much about it.
You have 2^(3/2) - sinx - cosx.
The important part is that 2^(3/2) > 2 and both sin(x) and cos(x) are at most 1.
(Greater than 2) - 2*(At most 1) is ALWAYS greater than 0. Same result.
You would have a problem if that constant were hanging around somewhere in the zone \(\displaystyle \sqrt{2} < Constant < 2\). If that were the case, you would need that result that I told you not to worry about. If the constant happened to be less than \(\displaystyle \sqrt{2}\), then there might be a triangle worth talking about.