In reply to Mantana Seeno.

The pdf of a multivariate uniform distribution is the constant 1/V where V is the volume of the region. This follows from the definition of a probability density. For example, on the unit square [0,1] x [0,1], the pdf is f(x,y)=1. Notices that if you integrate the density over the region, you obtain 1, which is the total probability. Similarly, on the rectangle [a,b] x [c,d], the pdf is f(x,y)=(b-a)(d-c). A reference would be any introductory text on probability or measure theory.