A graph of z = x^2 + xy + y^2. For the partial derivative at (1, 1) that leaves y constant, the corresponding tangent line is parallel to the xz-plane.

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2. Multivariate Finite Difference
3. A graph of z = x^2 + xy + y^2. For the partial derivative at (1, 1) that leaves y constant, the corresponding tangent line is parallel to the xz-plane.