An angle bisector is a line passing through the vertex of the angle that cuts the angle into two equal smaller angles.
If MN is angle bisector, then
m∠JMN=m∠NMK.
The two smaller angles are adjacent angles, then
m∠JMK=m∠JMN+m∠NMK=2m∠JMN.
Divide this equality by 2:
