Here's #19 . . .
Let19. is tangent to the circle and is a secant.
. . . FindCode:B x * * * - - - - - - * A * * / * * / 4 * * D / * / * * / * * / 5 * / * / * * / * * * C * * *
If a tangent and a secant are drawn to a circle from an external point,
the tangent is the mean proportional between the external segment
of the secant and the entire secant.