Partial Derivatives

Given a function of multiple variables the partial derivative is the tangent plane at a given input. Partial Derivatives are the inverse operation of multiple integrals.

revise above -- is it truly a plane or are they lines?
They use a similar notation to derivatives of one variable but use a slightly modified version, see partial derivative notation.

See Multivariable Calculus Notes - Chapter 14 - Sections 3-4 for more info about partial derivatives.

Finding Partial Derivatives

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When taking the partial derivative of a function you hold one of the variables as a constant and the other as a variable.