Prove that, Cos(alpha) + Cos(Beta) + Cos(Gamma) = 1

Given, Cos(Alpha) = Ax/A
Cos(Beta) = Ay/A
and, Cos(Gamma) = Az/A​

In chemistry,n molecules of an ideal gas have pressure, volume, and tempersture that stand inthe following relation, where r is constant. solve the equation for n,the number of molecules. pv=nrt