df(x,y,z)=∂f/∂xdx+∂f/∂ydy+∂f/∂zdz
=(1/z)[(x/y)^((1/z)-1)](1/y)dx+(1/z)[(x/y)^((1/z)-1)](-x/y^2)dy+[(x/y)^(1/z)]ln(x/y)(-1/z^2)dz
df(1,1,1)=dx-dy
df(x,y,z)=∂f/∂xdx+∂f/∂ydy+∂f/∂zdz
=(1/z)[(x/y)^((1/z)-1)](1/y)dx+(1/z)[(x/y)^((1/z)-1)](-x/y^2)dy+[(x/y)^(1/z)]ln(x/y)(-1/z^2)dz
df(1,1,1)=dx-dy