If (x+y)m+n=xmyn then dydx=
xy
−yx
−xy
yx
(m+n)log(x+y)=mlogx+mlogydydx=−(∂f/∂x)(∂f/∂y)=−m+nx+y−mxm+nx+y−my=−[nx−my]⋅y(my−nx)x=yx