Given: 1 and 2 are supplements, and 3 and 2 are supplements. prove: 1 3 complete the missing parts of the paragraph proof. by the definition of angles, the sum of the measures of angles 1 and 2 is 180 degrees. likewise, the sum of the measures of angles is 180 degrees. by the , m1 + m2 = m3 + m2. subtract the measure of angle from each side. you get m1 = m3, or 1 3, by the definition of congruence.