# Directional Derivative ![](Screen%20Shot%202021-05-13%20at%208.48.12%20AM.png) Here we are really looking at a limit of slopes of secant lines: ![](Screen%20Shot%202021-05-13%20at%208.50.41%20AM.png) Okay great, so this is our definition, but how do we compute it? Consider the following: * $f$ Depends on both $x$ and $y$ * $x$ and $y$ both depend on $s$ So we really have a composition of functions (see [here](https://www.youtube.com/watch?v=9yCtWfI_Vjg&list=PLHXZ9OQGMqxc_CvEy7xBKRQr6I214QJcd&index=18))! --- Links To: [Multivariable-Calculus](Multivariable-Calculus.md) [Partial-Derivatives](Partial-Derivatives.md) References: * [Directional Derivatives](https://www.youtube.com/watch?v=GJODOGq7cAY) * [Wikipedia](https://en.wikipedia.org/wiki/Directional_derivative) * [3b1b with Khan](https://www.khanacademy.org/math/multivariable-calculus/multivariable-derivatives/gradient-and-directional-derivatives/v/directional-derivative) * [formal defn, 3b1b with Khan](https://www.youtube.com/watch?v=4RBkIJPG6Yo&list=PLSQl0a2vh4HC5feHa6Rc5c0wbRTx56nF7&index=22) * [Directional derivatives and slope](https://www.youtube.com/watch?v=4tdyIGIEtNU)