Exterior angle
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An Exterior angle is the angle two lines form if one is extended as shown. In the figure, angle DCB is an exterior angle because its on the triangles exterior. it is supplementary to angle ACB.
![[asy]draw((0,50)--(40,1)--(0,0)--(0,50)); draw((0,50)--(0,68),linetype("0 2"),EndArrow); markscalefactor=0.4; draw(anglemark((40,1),(0,50),(0,68)),green); label("A",(0,0),W); label("B",(40,0),NE); label("C",(0,50),W); label("D",(0,68),NW); [/asy]](http://latex.artofproblemsolving.com/7/0/0/700140655ecb08f6316582e02d389bce562fdff0.png)
