Answer
True.
Work Step by Step
By the law of cosines: $c^2=a^2+b^2-2ab\cdot cos(C)$, in case of a right-angle triangle e.g. $C=90^o,$ $\cos{(90^o)}=0$, hence the law of cosines becomes $c^2=a^2+b^2-2ab\cdot0=a^2+b^2$, which is the Pythagorean Theorem. Hence it is true.
