Circle Ω has radius 5. Points A and B lie on Ω such that chord AB has length 6. A unit circle ω is tangent to chord AB at point T. Given that ω is also internally tangent to Ω, find AT⋅BT.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let M be the midpoint of chord AB and let O be the center of Ω. Since AM=BM=3, Pythagoras on triangle AMO gives OM=4. Now let ω be centered at P and say that ω and Ω are tangent at Q. Because the diameter of ω exceeds 1, points P and Q lie on the same side of AB. By tangency, O,P, and Q are collinear, so that OP=OQ−PQ=4. Let H be the orthogonal projection of P onto OM; then OH=OM−HM=OM−PT=3. Pythagoras on OHP gives HP2=7. Finally, AT⋅BT=AM2−MT2=AM2−HP2=9−7=2
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