How can one prove that any two right angles are congruent? The defintion of right angles has changed from 90 degrees: two lines l1 and l2 intersect at right angles if two adjacent angles are congruent. Congruence: points are congruent if there's an isometry which maps one onto the other.
I have a really hard time saying the right thing when writing these obvious proofs. And isometries throw me off. Uggghhh....
Thanks for any help!