Let XY be a tangent to a circle, and let XBA be a secant of the circle, as shown below. If AX = 15 and XY = 9, then what is AB?
XY · XY = XA · XB
9 · 9 = 15 · XB
XB = 81 / 15 = 5.4
AX = AB + BX
15 = AB + 5.4
AB = 9.6
