Maths Olympiad Prep

Track / Stage 3 / 63 of 260 #63 of 1964

Problem 63

AMC 10/12, early questions
Geometry Difficulty 3.2 Multiple choice

Let BCBC of right triangle ABCABC be the diameter of a circle intersecting hypotenuse ABAB in DD.
At DD a tangent is drawn cutting leg CACA in FF. This information is not sufficient to prove that

Pick one

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Official solution

We will prove every result except for B\fbox{B}.
By Thales' Theorem, ∠CDB=90∘\angle CDB=90^\circ and so ∠CDA=90∘\angle CDA= 90^\circ. FCFC and FDFD are both tangents to the same circle, and hence equal. Let ∠CFD=α\angle CFD=\alpha. Then ∠FDC=180∘−α2\angle FDC = \frac{180^\circ - \alpha}{2}, and so ∠FDA=α2\angle FDA = \frac{\alpha}{2}. We also have ∠AFD=180∘−α\angle AFD = 180^\circ - \alpha, which implies ∠FAD=α2\angle FAD=\frac{\alpha}{2}. This means that CF=DF=FACF=DF=FA, so DFDF indeed bisects CACA. We also know that ∠BCD=90−180∘−α2=α2\angle BCD=90-\frac{180^\circ - \alpha}{2}=\frac{\alpha}{2}, hence ∠A=∠BCD\angle A = \angle BCD. And ∠CFD=2∠A\angle CFD=2\angle A as α=α2×2\alpha = \frac{\alpha}{2}\times 2.
Since all of the results except for BB are true, our answer is B\fbox{B}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.