If the results of a random experiment are infinitely uncountable, and the possibility of each result is consistent, and every basic event in the sample space can be described by a bounded region, then this random experiment is called geometric probability. For any event a, p (a) = l (a)/l (ω). Where l is the geometric dimension (length, area, volume)

From the title, we can see the geometric probability, so the probability is the ratio of two areas. If you still don't understand why, you can refer to example 2.22 on page 525, "It falls without inclusion. . . The probability is proportional to the length of the subinterval. " Geometric probability is also used here. This is easy to understand by drawing a number axis. It is also a truth to look back at the area after knowing the length.

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