To prove that the sum of the measures of interior angles of triangle PQR is equal to 180°, Alex could write the following true statement:
Angle PQR measures 60°.
To see why this statement is true, we can use the fact that the sum of the measures of interior angles of a triangle is always 180°.
Let's start by finding the measure of angle PRQ. Since line n is parallel to line m, we know that angle PQM (which is opposite angle PRQ) is also 40°. Therefore, angle PRQ measures:
angle PRQ = 180° - angle PQM - angle QPR
angle PRQ = 180° - 40° - 80°
angle PRQ = 60°
Next, we can find the measure of angle PQR by using the fact that the sum of the measures of interior angles in a triangle is 180°. Since we know that angles QPR and PRQ add up to 80° + 60° = 140°, we have:
angle PQR = 180° - angle QPR - angle PRQ
angle PQR = 180° - 80° - 60°
angle PQR = 40°
Therefore, we have shown that the sum of the measures of interior angles of triangle PQR is equal to 180°, and that angle PQR measures 40°.