Tangent Lines

Suppose to co-planar circles C_1 and C_2 have no points in common. Determine how many lines, if any, are tangent to both C_1 and C_2?
Source: NCTM Mathematics Teacher, October 2006

Solution

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When one circle is inside the other, there are no tangent lines to both circles. When the two circles are side by side, there are exactly four tangent lines to both circles.

Answer: 0 or exactly 4

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About mvtrinh

Retired high school math teacher.
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