Triangle it Out!!!!

Geometry Level 3

In A B C \triangle ABC D ~~D is the midpoint of side B C BC and A C B = 30 . A D B = 45 \angle ACB= { 30 }^{ \circ }.\quad \angle ADB=45 Find B A D . \angle BAD.


The answer is 30.

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2 solutions

Harrison Wang
Jan 7, 2015

Drop an altitude from A to BC with foot F. Let AF equal x. This means D F = x DF =x , C F = x 3 CF =x\sqrt3 , C D = x 3 x \rightarrow CD=x\sqrt3-x . Since D is the midpoint of CB, then C B = x 3 x CB = x\sqrt3-x and D F = 2 x x 3 DF=2x-x\sqrt3 . Therefore tan D A F = 2 3 \tan\angle DAF =2-\sqrt3 . It can easily be derived from half-angle identities that B A F = 1 5 \angle BAF =15^\circ . So our desired B A D = 45 15 = 3 0 \angle BAD =45-15=\boxed{30^\circ}

Bill Bell
Jan 10, 2015

Mr Wang's solution is more elegant than mine, definitely.

This prints the angles of ABC to be 30, 45 and 105 respectively from which the rest follows easily.

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