Let X1 and X2 be independent chi-square random variables with r1 and r2 degrees of freedom, respectively. Show that:

a) U = X1/(X2 + X1) has a beta distribution with alpha = r1/2 and beta = r2/2

b) V = X2/(X2 + X1) has a beta distribution with alpha =r2/2 and B = r1/2

How do you even start this? I need a lot of help with this one.