If C1, C2 be the centres of two circles and r1, r2 be the respective radii such that the distance between the centres is equal to the sum of the radii of the two circles, find the number of common tangents.

4 2 1 3

3

If C1, C2 = r1 and r2 then given two circles touch each other internally. Hence number of common tangents to the given circles in this case is 1.