Answer: 12
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Explanation:
Let's start with what the hint gives us. So the first sub-goal is to prove triangle ABE is similar to triangle DCE.
Since points A and D are points of tangency, this means the radii of each of those circles is perpendicular to the common internal tangent. So angles EAB and EDC are 90 degrees each.
Due to the vertical angle theorem, we also know that angles AEB and DEC are the same (we don't know the measure but we know they're equal angles).
So we have two pairs of congruent corresponding angles between the triangles, which is sufficient to let us use the AA (angle angle) similarity theorem. Therefore, the triangles have been proven to be similar. Triangle DCE is a reduced scaled down copy of triangle ABE. Or in reverse, triangle ABE is an enlarged copy of triangle DCE.
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Since the triangles are similar, we can form the proportion below and solve for x
AB/AE = DC/DE
x/18 = 4/6
x*6 = 18*4 .... cross multiplication
6x = 72
x = 72/6
x = 12
Therefore, segment AB is 12 units long, and this is the radius of circle B.
