Two perpendicular lines form four congruent angles.
1. Lines l and m are perpendicular. 1. Given.
2. Lines l and m form a right angle. 2. Definition of perpendicular.
3. Lines l and m form a 90 degree angle. 3. Definition of right angle.
Call it / 1.
4. The angles adjacent to / 1
are each 4. Postulate11.
90 degrees. Call them /
2 and / 3.
5. The fourth angle, call it / 4,
adjacent 5. Postulate
11.
to / 2
and / 3 is 90 degrees.
6. / 1, / 2, / 3, and / 4 are right angles. 6. Definition of right angle.