Question 642235
I guess, you need a proof.

{{{drawing( 300, 200,  0.0, 7.5,  -1.5, 3.0, 
            line( 0.5, 0.5, 6.5,  0.5), 
            line( 0.5, 0.5, 3.0,  2.5),
            line( 3.0, 2.5, 5.25, -0.5),

            locate(0.5, 0.5, A),
            locate(3.0, 2.9, B),
            locate(4.3, 0.5, C),

            locate(6.5,  0.5, D),
            locate(5.05, -0.5, E),

            arc (0.5, 0.5, 0.8, 0.8, 320, 360),

            arc (3.0, 2.5, 0.8, 0.8, 50,  140),
            arc (3.0, 2.5, 1.0, 1.0, 50,  140),

            arc (4.5, 0.5, 0.8, 0.8, 230, 360),
            arc (4.5, 0.5, 0.9, 0.9, 230, 360),
            arc (4.5, 0.5, 1.0, 1.0, 230, 360),

            arc (4.5, 0.5, 0.8, 0.8, 50, 180),
            arc (4.5, 0.5, 0.9, 0.9, 50, 180),
            arc (4.5, 0.5, 1.0, 1.0, 50, 180)
)}}}


1. angle ACE ie exterior angle of triangle ABC.....| given
2. mA+mB+mACB=180..............................................|the sum theorem
3. m ACE +m ACB=180........................................|def. of linear pair
4. mA+mB+mACB=m ACE +mACB........................|transitive property
5. mA +mB=m ACE.........................................|subtraction property