设 x/y=ln(y/x) ,求 dy/dx
问题描述：
设 x/y=ln(y/x) ,求 dy/dx
答
x/y=ln(y/x)
x(-1/y^2)y' +1/y = x/y( -y/x^2 +y'/x)
(1/y+x/y^2)y' = 1/y + 1/x
[(y+x)/y^2]y' = (x+y)/xy
y' = y/x