The joint cumulative distribution function (CDF) of R (radius) and θ (angle) is given by:
F(R, θ) = (1/2) * (r^2) * θ / π
Since the dart is equally likely to land at any point inside a circular target of unit radius, the probability of the dart landing within a region A is proportional to the area of A on the unit circle.
This formula represents the probability of R being less than or equal to r and Θ being less than or equal to θ, considering a circular target of unit radius.
The expression holds for 0 ≤ r ≤ 1 and 0 ≤ θ ≤ 2π. It is derived from the ratio of the area of the circular sector defined by R ≤ r and Θ ≤ θ to the total area of the unit circle, reflecting the equal likelihood of the dart landing at any point within the circular target.