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On a two-variable surface, partial derivatives represent the slopes of the tangent plane in the coordinate directions. The partial derivative with respect to $x$ is the slope along the $x$-direction, and the partial derivative with respect to $y$ is the slope along the $y$-direction.

Conditions: The surface is locally smooth enough to have a tangent plane.; The variables are distinguished as $x$ and $y$.

On a two-variable surface, partial derivatives represent the slopes of the tangent plane in the coordinate directions. The partial derivative with respect to $x$ is the slope along the $x$-direction, and the partial derivative with respect to $y$ is the slope along the $y$-direction.

Conditions: The surface is locally smooth enough to have a tangent plane.; The variables are distinguished as $x$ and $y$.