Circles , centre , and , centre , touch externally at . A third circle, , which contains and , touches and at and , respectively. Point is joined to and the line is extended to meet at .
Prove that and intersect on the circumference of .
Solutions — 5
Solution 1
Let be the centre of . Then, , , are collinear as are , , . Let be the intersection point of the lines and .

Because and , we have , hence is parallel to . Using that , , , are collinear and we now get hence triangle is isosceles with and so is on .
Alternatively, we may consider , which has a right angle at . Because the centre, , of is on , this implies that must be on .
Solution 2
Draw tangents at and to and let them intersect at . The radical axis of two circles that are tangent to each other is their common tangent. Therefore, is the radical centre of , and , which implies that is the common tangent of and . Hence, is perpendicular to . Let be the second intersection point of the line with , the intersection point of the lines and , and the intersection point of and . From we obtain
Considering the angle sum in we see that , hence .
Considering the external angle at in , we obtain , i.e. .
The Alternate Segment Theorem applied to circle gives us
Similarly, when we consider the external angle at in we can use to obtain .
We can now conclude in two different ways. One way is to consider the triangles and which have an angle in common at . Because , we see that . Hence is perpendicular to and so . This implies that
hence is on by the converse of the Alternate Segment Theorem.
Alternatively, we may consider , which has a right angle at . Because the centre, , of is on , this implies that must be on .
Solution 3
We use the diagram and notations from Solution 2 and let be the centre of . Then the homothety of centre sending to sends to and the homothety of centre sending to sends to . Since , , are collinear, it follows that , , are collinear hence . We conclude as in Solution 2.
Solution 4
Extend to the diameter of circle and let be the second intersection point of with . Let be the centre of .

Consider the angle between and the common tangent to and at . The Alternate Segment Theorem for these circles then yields
Note that as well as . Because , we get
Considering the external angle of triangle at , we get
Subtracting (3) from (4), we obtain now . Together with we now see that
If we combine this with (2), we get
If denotes the second intersection point of and , we have , i.e. is a diameter of and so is the intersection point of and .
Solution 5
Let be the intersection point of the lines and . Lines and meet at the centre of circle .

From we see that . The central angle stands on the same arc of as , hence . Considering external angles of triangles and we obtain
and this implies that is on .