The is equal to . The bisectors and the exterior bisectors of the angles define the yellow quadrilateral.
What are the angles of this quadrilateral?
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Call the intersection of the interior angle bisectors I and the intersection of the exterior angle bisectors E . Let X be an arbitrary point on A B to the right of B .
Since I B is the angle bisector of ∠ A B C , ∠ I B C = 2 1 ∠ A B C .
Since I E is the angle bisector of ∠ C B X , ∠ C B E = 2 1 ∠ C B X = 2 1 ( 1 8 0 − ∠ A B C ) = 9 0 − 2 1 ∠ A B C .
Thus, ∠ I B E = ∠ I B C + ∠ C B E = 2 1 ∠ A B C + ( 9 0 − 2 1 ∠ A B C ) = 9 0 ∘ . A similar process also shows that ∠ I C E = 9 0 ∘ . This narrows our choices down to either the first or the last one.
Now, we shall find ∠ B I C . Since ∠ I B C = 2 1 ∠ B and ∠ I C B = 2 1 ∠ C , we have
∠ B I C = 1 8 0 − ( ∠ I B C + ∠ I C B ) = 1 8 0 − 2 1 ( ∠ B + ∠ C ) = 1 8 0 − 2 1 ( 1 8 0 − ∠ A ) = 9 0 + 2 1 ∠ A = 9 0 + 2 1 ( 2 0 ) = 1 0 0 ∘ .
Thus, we conclude that B I C E has angles 1 0 0 ∘ , 9 0 ∘ , 9 0 ∘ , 8 0 ∘ .