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 property, m1 + m2 = m3 + m2. Subtract the measure of angle from each side. You get m1 = m3, or 1 3, by the definition of congruence.