A Math Futher Problems on Locus

In both of the two questions, a theorem in Geometry is used:

When two circles touch each other, the point of contact lies on the st. line joining the two centres.=====

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圖片參考：http://i277.photobucket.com/albums/kk75/uncle_michael/080713_mat02b.jpg?t=1215954937


2008-07-13 21:20:46 補充：
In 9, there is a typing mistake.

the answer should be x^2 + 2ry - r^2 = 0