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## Homework Statement

The changing variable formula in partial derivative

f(u,v)

x=x(u,v)

y=y(u,v)

(∂f/∂x)

_{y}= (∂f/∂u)

_{v}(∂u/∂x)

_{y}+ (∂f/∂v)

_{u}(∂v/∂x)

_{y}

I khow the how chain rule works, but I don't know why in the (∂f/∂u) v is constant and in the (∂u/∂x) y is constant