Given: bisects ∠aec. ∠aed is a straight angle. prove: m∠aeb = 45° complete the paragraph proof. we are given that bisects ∠aec. from the diagram, ∠ced is a right angle, which measures degrees. since the measure of a straight angle is 180°, the measure of angle must also be 90° by the. a bisector cuts the angle measure in half. m∠aeb is 45°