in proving a beta gamma relation.

[swaha]

B(m,n)=gamma(m).gamma(n)/gamma(m+n)

while changing the variables x to rcosz, y=rsinz and taking their differentials we tactly assume the terms containing (dz)^2 & (dr)^2 terms vanish. we only consider terms containing dzdr in the double integral in place of dxdy can somebody please explain why (dz)^2 & (dr)^2 terms vanish or not considered for the double integral?

[Bignose]

I think you need to provide more detail. You have B(m,n) and then you have variables x, y, z, and r? I'm confused...