Until you have a definition of right angles, it is not possible to prove that anything, including the angle opposite the 5 side of a 3,4,5 triangle, is a right angle. Really, that's what a definition is.
It isn't necessary for the statement "all right angles are equal" to include a definition of right angles. But for the statement to have any meaning at all, the logical system that the statement is made in must include a definition of right angles. They cannot be left undefined.
It isn't necessary for the statement "all right angles are equal" to include a definition of right angles. But for the statement to have any meaning at all, the logical system that the statement is made in must include a definition of right angles. They cannot be left undefined.