Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/math/multivariable-calculus/multivariable-derivatives/partial-derivatives-of-vector-valued-functions/v/partial-derivative-of-a-parametric-surface-part-1 When a vector-valued function represents a parametric surface, how do you interpret its partial derivative?