Given: Angle1 and Angle2 are supplements, and Angle3 and Angle2 are supplements. Prove: Angle1 Is-congruent-to Angle3 Three separate angles are shown. They are labeled 1, 2, 3 from left to right. 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, mAngle1 mAngle2 = mAngle3 mAngle2. Subtract the measure of angle from each side. You get mAngle1 = mAngle3, or Angle1 Is-congruent-to Angle3, by the definition of congruence.