# Constructing a circle that internally tangents a circle $\gamma$ and passes through two internal points.

The full details of this problem is given as follows

Construct a circle $\gamma$ with center $O_\gamma$ , and place two points $A$ and $B$ inside $\gamma$. That does not lie on the edge of the circle. Explain the construction of a point $C$, such that the circle $ABC =\beta$, is internally tangential to $\gamma$.

Now $ABC$ means a circle that passes through the points $A$,$B$ and $C$. I have made a drawing, but I am unable to mathematicaly construct the point $C$. I already know that for most pairs $A$,$B$ there are two possible choices for $C$. Eg $C_1$ and $C_2$. See the following figure

Can anyone show me or help me in finding the placement of $C$, given $A$ and $B$? The figure is only but a sketch, but I know that the centre of the circle obviously has to lie on the perpendicular bisector of A and B, after that I am clueless.

• I'd say that the tangent in C is perpendicular to both $O_\beta$ and $O_\gamma$? – long tom Mar 21 '13 at 10:33
• So $C$, $O_\gamma$ and $O_\beta$ are collinear? – long tom Mar 21 '13 at 10:36
• i think the answer might be on math.stackexchange.com/questions/32386/… (last part of accepted answer) – long tom Mar 21 '13 at 10:53
• You're looking for a circle that is tangent to two given circles? – Sgernesto Mar 22 '13 at 12:52
• Given two points A and B inside a circle $\gamma$. Find a point $C$ that lies on $\gamma$, such that the circle that passes through $A$,$B$,$C$ only touches $\gamma$ at $C$. folk.ntnu.no/oistes/Diverse/sirkelmaple.pdf – N3buchadnezzar Mar 22 '13 at 13:05