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