Answer:
UD = 20
Explanation:
Given a tangent and a secant from an external point to the circle, then
The square of the tangent is equal to the product of the external part and the entire secant, that is
12² = 7.2(7.2 + CD)
144 = 7.2² + 7.2CD
144 = 51.84 + 7.2CD ( subtract 51.84 from both sides )
92.16 = 7.2CD ( divide both sides by CD )
12.8 = CD
Thus
UD = UC + CD = 7.2 + 12.8 = 20