Similar triangles?

Geometry Level 3

The B A C \angle BAC is equal to 20 ° 20° . The bisectors and the exterior bisectors of the A B C , A C B \angle ABC, \angle ACB angles define the yellow quadrilateral.

What are the angles of this quadrilateral?

120°, 80°, 80°, 80° 100°, 100°, 80°, 80° 100°, 90°, 90°, 80° 110°, 100°, 90°, 60° None of the others

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1 solution

Steven Yuan
Aug 8, 2017

Call the intersection of the interior angle bisectors I I and the intersection of the exterior angle bisectors E . E. Let X X be an arbitrary point on A B AB to the right of B . B.

Since I B IB is the angle bisector of A B C , \angle ABC, I B C = 1 2 A B C . \angle IBC = \frac{1}{2} \angle ABC.

Since I E IE is the angle bisector of C B X , \angle CBX, C B E = 1 2 C B X = 1 2 ( 180 A B C ) = 90 1 2 A B C . \angle CBE = \frac{1}{2} \angle CBX = \frac{1}{2}(180 - \angle ABC) = 90 - \frac{1}{2} \angle ABC.

Thus, I B E = I B C + C B E = 1 2 A B C + ( 90 1 2 A B C ) = 9 0 . \angle IBE = \angle IBC + \angle CBE = \frac{1}{2} \angle ABC + \left ( 90 - \frac{1}{2} \angle ABC \right ) = 90^{\circ}. A similar process also shows that I C E = 9 0 . \angle ICE = 90^{\circ}. This narrows our choices down to either the first or the last one.

Now, we shall find B I C . \angle BIC. Since I B C = 1 2 B \angle IBC = \frac{1}{2} \angle B and I C B = 1 2 C , \angle ICB = \frac{1}{2} \angle C, we have

B I C = 180 ( I B C + I C B ) = 180 1 2 ( B + C ) = 180 1 2 ( 180 A ) = 90 + 1 2 A = 90 + 1 2 ( 20 ) = 10 0 . \begin{aligned} \angle BIC &= 180 - (\angle IBC + \angle ICB) \\ &= 180 - \dfrac{1}{2}(\angle B + \angle C) \\ &= 180 - \dfrac{1}{2}(180 - \angle A) \\ &= 90 + \dfrac{1}{2} \angle A \\ &= 90 + \dfrac{1}{2} (20) \\ &= 100^{\circ}. \end{aligned}

Thus, we conclude that B I C E BICE has angles 10 0 , 9 0 , 9 0 , 8 0 . \boxed{100^{\circ}, 90^{\circ}, 90^{\circ}, 80^{\circ}}.

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