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, m Angle [image 1] 1 + m Angle [image 1] 2 = m Angle [image 1] 3 + m Angle [image 1] 2. Subtract the measure of angle [___] from each side. You get m Angle [image 1] 1 = m Angle [image 1] 3, or Angle [image 1] 1 Is-congruent-to [image 2] Angle [image 1] 3, by the definition of congruence.