CircleHardBloom L3

Question

Tangents are drawn from any point on the circle $x^{2} + y^{2} = R^{2}$ to the circle $x^{2} + y^{2} = r^{2}$. If the line joining the points of intersection of these tangents with the first circle also touch the second, then $R$ equals:

Options

A.$\sqrt{2}r$
B.$2r$
C.$\frac{2r}{2 - \sqrt{3}}$
D.$\frac{4r}{3 - \sqrt{5}}$

Solution

[Key Concept: Tangents from a point on $x^{2} + y^{2} = R^{2}$ to $x^{2} + y^{2} = r^{2}$: the chord of contact has a fixed length.]

$r = Rcos60^{\circ} = \frac{R}{2}$

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