1) given 2) definition of linear pair 3) definition of linear pair and supplementary 4) definition of supplementary supplementary angles add to 180 5) the measure of angle one plus the measure of angle 2 plus the measure of angle 3 equals 180 6) substitution in lines 4 and 5 7) the measure of angle one plus the measure of angle 2 equals measure of angle 4 by algebra and simplification
