The secant and tangent theorem states:
If a tangent segment and a secant segment are drawn to a circle from an exterior point, then the square of the measure of the tangent segment is equal to the product of the measures of the secant segment and its external secant segment.
In the given figure DE is tangent and AD is the secant .Applying the THeorem we have:
BD.AB=DE^{2}
Substituting the values from figure
10(a+10)=12^{2}
