Here, I'm going to show the way to solve this mathematical problem by vector.
At first, I define

,

and

as a vector which starts at the center of each circle and finishes at the center of another circle like diagram below.
Next, I define

and

as a vector which starts at the point of intersection of two tangent of particular set of two circles and finishes different point of the similar intersection.
And, I define radius of each circle as

,

and

.
The important point of this problem is to describe

by

, that is, to prove that

is on the same line as

.
Then, how we can describe

by

.
Please think about describe

and

in different formula, that is, the formula using

,

,

,

,

and

because if we can describe

and

by same components, we may find new relationship between

and

.