Let BC of right triangle ABC be the diameter of a circle intersecting hypotenuse AB in D. At D a tangent is drawn cutting leg CA in F. This information is not sufficient to prove that
We will prove every result except for B. By Thales' Theorem, ∠CDB=90∘ and so ∠CDA=90∘. FC and FD are both tangents to the same circle, and hence equal. Let ∠CFD=α. Then ∠FDC=2180∘−α, and so ∠FDA=2α. We also have ∠AFD=180∘−α, which implies ∠FAD=2α. This means that CF=DF=FA, so DF indeed bisects CA. We also know that ∠BCD=90−2180∘−α=2α, hence ∠A=∠BCD. And ∠CFD=2∠A as α=2α×2. Since all of the results except for B are true, our answer is B.
Source: NuminaMath-1.5,
licensed Apache-2.0.
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