You still haven't stated clearly what you are trying to do. Please state what you are given (e.g. the side lengths), what you want to find (e.g. the...
Okay, context can make a big difference. If you were a student of multivariable calculus, I would expect you to prove everything. If this makes sense...
I was a software engineer for many years, and have been teaching at a community college since 2004. Since 1998, I was a part of Ask Doctor Math (http://mathforum.org/dr.math/ and now http://themathdoctors.org/).