what's the formula for calculating the diagonal of cube? i m confused..whether it is ''a under root 2''...or ''a under root 3''...????

i m stiill confused b'cos...suppose if we take the 2 sides and the diagonal of cube than it forms the right triangle...then when i m applying the pythagoras theorem to find the diagonal..than i m getting the same ans.as when applied the formula ''a under root 2''....not the ''a under root 3''.....