# Directional Derivative of g(x, y) = sqrt(x^2 + y^2) at (3, 4) in the direction of v = 8i - 15j | Summary and Q&A

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October 25, 2017
by
The Math Sorcerer
Directional Derivative of g(x, y) = sqrt(x^2 + y^2) at (3, 4) in the direction of v = 8i - 15j

## TL;DR

This video explains how to calculate the directional derivative of a function at a specific point in the direction of a given vector.

## Key Insights

• π₯ The directional derivative of a function at a point measures the rate of change of the function in a specific direction.
• π¦πͺ Finding a unit vector in the direction of a given vector is crucial for calculating the directional derivative accurately.
• βΊοΈ The formula for the directional derivative involves partial derivatives with respect to X and Y, as well as cosine and sine of the angle between the given vector and positive X-axis.
• β Computing the partial derivatives involves using the chain rule and treating the function as something to the one-half power.
• π¦πͺ The Pythagorean theorem is used to calculate the magnitude of a vector, which is necessary for finding a unit vector.

## Transcript

so in this video we're going to find the directional derivative of this function at this point in the direction of this vector the formula we're going to use for the directional derivative it's a directional derivative the direction of the unit vector U of XY as equal to the partial with respect to X and X Y cosine theta plus the partial with respe... Read More

### Q: What is the formula for calculating the directional derivative of a function in a given direction?

The formula is: directional derivative in the direction of vector U = partial derivative with respect to X times cosine(theta) + partial derivative with respect to Y times sine(theta).

### Q: How do we find a unit vector in the direction of a given vector?

To find a unit vector in the direction of a given vector V, divide V by its magnitude, which is calculated using the Pythagorean theorem (square root of the sum of squared components).

### Q: How do we compute the partial derivatives of a function?

To compute the partial derivative with respect to X, treat the function as something to the one-half power and use the chain rule. For the partial derivative with respect to Y, follow the same process but differentiate with respect to Y.

### Q: Can you provide an example of calculating the directional derivative?

Sure! Let's calculate the directional derivative of the function at the point (3, 4) in the direction of the vector (8, -15). By plugging in the values, simplifying the expression, and performing the necessary computations, we can find the directional derivative.

## Summary & Key Takeaways

• The video teaches how to calculate the directional derivative of a function at a specific point using the formula involving partial derivatives.

• It explains the importance of finding a unit vector in the direction of a given vector and demonstrates how to calculate it.

• The video shows the steps to compute the partial derivatives of the function and provides an example of plugging in values to find the directional derivative.