Three Circles

Consider the following figure:

If the radius of the blue quarter-circle is 2, which is also the diameter of the larger half-circle (colored pink), then what is the radius of the smaller half-circle (colored yellow)?

Source: unknown

Click here for the answer.

Internet references

Wikipedia: Descartes' theorem 

Mathworld: Tangent Circles 

Related pages in this website

Descartes' Circle Theorem

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