Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Find the answer

Circle Ω\Omega has radius 5. Points AA and BB lie on Ω\Omega such that chord ABA B has length 6. A unit circle ω\omega is tangent to chord ABA B at point TT. Given that ω\omega is also internally tangent to Ω\Omega, find ATBTA T \cdot B T.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let MM be the midpoint of chord ABA B and let OO be the center of Ω\Omega. Since AM=BM=3A M=B M=3, Pythagoras on triangle AMOA M O gives OM=4O M=4. Now let ω\omega be centered at PP and say that ω\omega and Ω\Omega are tangent at QQ. Because the diameter of ω\omega exceeds 1, points PP and QQ lie on the same side of ABA B. By tangency, O,PO, P, and QQ are collinear, so that OP=OQPQ=4O P=O Q-P Q=4. Let HH be the orthogonal projection of PP onto OMO M; then OH=OMHM=OMPT=3O H=O M-H M=O M-P T=3. Pythagoras on OHPO H P gives HP2=7H P^{2}=7. Finally, ATBT=AM2MT2=AM2HP2=97=2A T \cdot B T=A M^{2}-M T^{2}=A M^{2}-H P^{2}=9-7=2

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