August 2018 Geometry#3

Geometry Level pending

If A F C B AF\perp CB , C D A B CD\perp AB , B E A C BE \perp AC ,

And D F B = 63 ° \angle DFB = 63° , D C B = 31 ° \angle DCB = 31° , then find E D F \angle EDF (in degrees).


The answer is 64.

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1 solution

Let H = A F C D B E H=AF \cap CD \cap BE

Claim: H H is the incenter of Δ D E F \Delta DEF

Proof:

Note that E H F C , D H F B EHFC, DHFB and E D B C EDBC are cyclic.

So E F H = E C D = D B E = D F H \angle EFH = \angle ECD = \angle DBE = \angle DFH

Similarly for other angles, we obtain that H I . H \equiv I.

Note that D B E = 2 7 \angle DBE = 27^{\circ} and D B C = 5 9 \angle DBC = 59^{\circ}

So E B C = C A F = E D C = C D F = 3 2 \angle EBC = \angle CAF = \angle EDC = \angle CDF = 32^{\circ}

m E D F = 64 . \therefore m\angle EDF = \boxed{64}.

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