Complete the indirect proof to show that two supplementary angles cannot both be obtuse angles. given: angle1 and angle2 are supplementary. prove: angle1 and angle2 cannot both be obtuse. assume that two supplementary angles can both be obtuse angles. so, assume that angle1 and angle2 are obtuse. then mangle1 > 90° (equation 1) and mangle2 > ° (equation 2) by the definition of angles. adding the two inequalities, mangle1 + mangle2 > ° (equation 3). however, by the definition of supplementary angles, mangle1 + mangle2 = (equation 4). so equation 3 contradicts the given information. this means the assumption is , and therefore angle1 and angle2 both be obtuse.