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← JEE Advanced 2019 · Paper 1
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JEE Advanced 2019 · Paper 1, Mathematics question 16

Let the point BB be the reflection of the point A(2,3)A(2, 3) with respect to the line 8x−6y−23=08x - 6y - 23 = 0. Let ΓA\Gamma_A and ΓB\Gamma_B be circles of radii 2 and 1 with centres AA and BB respectively. Let TT be a common tangent to the circles ΓA\Gamma_A and ΓB\Gamma_B such that both the circles are on the same side of TT. If CC is the point of intersection of TT and the line passing through AA and BB, then the length of the line segment ACAC is ______

Answer and worked solution

Official answer: 10

Reflect A. At A(2,3)A(2,3), 8x−6y−23=−258x-6y-23=-25, so
B=(2,3)−2(−25)100(8,−6)=(6,0),AB=5.B=(2,3)-\frac{2(-25)}{100}(8,-6)=(6,0),\qquad AB=5.

Where T meets AB. With both circles on the same side, TT is a direct common tangent. It meets line ABAB at the external centre of similitude CC, where CACB=21\frac{CA}{CB}=\frac21 and CC lies beyond BB:
CA−CB=5,CA=2CB  ⟹  CA=10.CA-CB=5,\qquad CA=2CB\implies CA=10.
Answer: 10