1. m <3 = m <4, GIVEN
2. <2, <3 supplementary, SAME SIDES IN INTERSECTING RAYS
3. m <2 + m <3 = 180, DEF. OF SUPPLEMENTARY ANGLES
4. SUBSTITUTION
5. m <1 = m <4, VERTICAL (OPPOSITE) ANGLES ARE CONGRUENT
6. m <2 + m <1 = 180, #4 SUBSTITUTION
7. <1, <2 supplementary, DEF. OF SUPPLEMENTARY ANGLES
