Right triangle and probability function

[BatyrzhanQZ]

Particles B1B2 and F1F2 designate the ends of the legs (cathets). By varying length of one cathet we change the length of the other, bringing points B1B2 and F1F2 closer or farther to each other.

How changes in the length of the cathet F1F2 influence (show) the probability of finding particles B1B2 closer to each other?

Please describe this probability algebraically, trigonometrically.