Explanation:
1.Given .
2.Being vertically opposite angle.
3.Because sum of all interior angle of a triangle is always 180°.
4.From the statement of 3 number.
5.Because angle 4 = angle 3 and angle 6= angle 1 .
6.(suppose ,< is angle sign) solving this equation,
m<1 + m<2+ m<3 = m<3 + m<5 + m<1