If two circles touch one another, then they do not have the same center. | ||
Let the two circles ABC and CDE touch one another at the point C.
I say that they do not have the same center. For, if possible, let it be F. Join FC, and draw FEB through at random. |
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Then, since the point F is the center of the circle ABC, FC equals FB. Again, since the point F is the center of the circle CDE, FC equals FE. | I.Def.15 | |
But FC was proved equal to FB, therefore FE also equals FB, the less equals the greater, which is impossible.
Therefore F is not the center of the circles ABC and CDE. | ||
Therefore if two circles touch one another, then they do not have the same center. | ||
Q.E.D. |
This propostion is not used in the rest of the Elements.
Book III Introduction - Proposition III.5 - Proposition III.7.