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
.