CircleHardBloom L3

Question

A square $OABC$ is formed by line pairs $xy = 0$ and $xy + 1 = x + y$ where ' $O$ 'is the origin. A circle with centre $C_{1}$ inside the square is drawn to touch the line pair $xy = 0$ and another circle with centre $C_{2}$ and radius twice that of $C_{1}$, is drawn to touch the circle $C_{1}$ and the other line pair. The radius of the circle with centre $C_{1}$ is:

Options

A.$\frac{\sqrt{2}}{\sqrt{3}(\sqrt{2} + 1)}$
B.$\frac{2\sqrt{2}}{3(\sqrt{2} + 1)}$
C.$\frac{\sqrt{2}}{3(\sqrt{2} + 1)}$
D.$\frac{\sqrt{2} + 1}{3\sqrt{2}}$

Solution

[Key Concept: From the line-pairs $xy = 0$ and $xy + 1 = x + y$ find the square's vertices, then read the property.]

$C_{1},C_{2}$ lie on $y = x$ line

$$\begin{matrix} OB = OC_{1} + C_{1}C_{2} + \\ \sqrt{2} = r\sqrt{2} + 3r + 2r \\ \ r = \frac{\sqrt{2}}{3(\sqrt{2} + 1)} \end{matrix}$$

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