can you use pythagoras on a non right angled triangle

Question

Sorry I'm a bit confuse, are you allow to use Pythagoras in a non right angled triangle?

Answer 1

The Pythagorean theorem is a restriction of the law of cosines. Given triangle ABC with sides $a,b,c$ opposite the corresponding angles we have $$a^2=b^2+c^2-2bc\cos A$$ As $\cos 90^\circ=0$ the last term drops out in a right triangle.

Answer 2

No, you can't.

If that were the case, then an equilateral triangle with sides $1, 1, 1$ would have $1^2 = 1^2 + 1^2$, which is not true.