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Geometry Level 3

In a quadrilateral A B C D ABCD , D A B = A B C = 11 0 , B C D = 3 5 \angle DAB = ∠ABC = 110^{\circ }, \angle BCD = 35^{\circ} , and A C AC bisects D A B \angle DAB .

Find the measure of A B D \angle ABD in degree measures.


The answer is 40.

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2 solutions

Let the A D B A C O . \odot ADB \cap AC \equiv O. Now, A O AO is the bisector of A D B \angle ADB , therefore, A O AO bisects arc B D BD and B O = D O BO = DO . Also, D O B = 7 0 = > \angle DOB = 70^{\circ} => O O is the circumcenter of Δ D B C \Delta DBC .

Clearly, A O D + D O C = A B D + 2 × D B C = A B D + 2 × [ 11 0 A B D ] = 18 0 = > A B D = 4 0 \angle AOD + \angle DOC = \angle ABD + 2 \times \angle DBC = \angle ABD + 2 \times [\angle 110^{\circ} - \angle ABD] = 180^{\circ} => \angle ABD = 40^{\circ}

Ayush G Rai
Jul 8, 2017

Let I I be the Incenter of A B D \triangle ABD and A B D = 2 x . \angle ABD=2x. B I D = 90 + A 2 = 14 5 . \angle BID=90+\dfrac{\angle A}{2}=145^\circ. We can clearly see that Quad. B I D C BIDC is cyclic since B I D + B C D = 14 5 + 3 5 = 18 0 . \angle BID+\angle BCD=145^\circ+35^\circ=180^\circ. By angle chasing we know that A C D = 2 0 = D B I \angle ACD=20^\circ=\angle DBI by cyclic properties. Therefore A B D = 2 × 2 0 = 4 0 . \angle ABD=2\times 20^\circ=\boxed{40^\circ}.

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