When a tangent line (13.5 cm) and a secant (lines x + 8.45 cm) intersect then:
tangent line^2 = 8.45 * (8.45 + x)
13.5^2 = 71.4025 + 8.45 x
182.25 -71.4025 = 8.45x
8.45 x = 110.8475
x = 13.1180473373
x = 13.1 (rounded)
Source:
1728.com/circangl.htm
