Question

Second partial derivatives Find the four second partial derivatives of the following functions. $$f(x, y)=x^{2} \sin y$$

Short answer

Question: Compute the four second partial derivatives of the function $$f(x, y) = x^2 \sin y$$. Answer: The four second partial derivatives of the given function are: 1. $$f_{xx}(x, y) = 2 \sin y$$ 2. $$f_{yy}(x, y) = -x^2 \sin y$$ 3. $$f_{xy}(x, y) = 2x \cos y$$ 4. $$f_{yx}(x, y) = 2x \cos y$$

Step-by-step solution

Find the first partial derivatives

Firstly, we need to determine the first partial derivatives with respect to x and y: $$f_x(x, y) = \frac{\partial}{\partial x} (x^{2} \sin y) = 2x \sin y$$ $$f_y(x, y) = \frac{\partial}{\partial y} (x^{2} \sin y) = x^{2} \cos y$$

Find the second partial derivatives

Now, we will find the second partial derivatives using the first partial derivatives we found in step 1. 1. Second derivative with respect to x: $$f_{xx}(x, y) = \frac{\partial}{\partial x} (2x \sin y) = 2 \sin y$$ 2. Second derivative with respect to y: $$f_{yy}(x, y) = \frac{\partial}{\partial y} (x^{2} \cos y) = -x^{2} \sin y$$ 3. Second derivative with respect to x and y: $$f_{xy}(x, y) = \frac{\partial}{\partial y} (2x \sin y) = 2x \cos y$$ 4. Second derivative with respect to y and x: $$f_{yx}(x, y) = \frac{\partial}{\partial x} (x^{2} \cos y) = 2x\cos y$$ The four second partial derivatives are: 1. $$f_{xx}(x, y) = 2 \sin y$$ 2. $$f_{yy}(x, y) = -x^{2} \sin y$$ 3. $$f_{xy}(x, y) = 2x \cos y$$ 4. $$f_{yx}(x, y) = 2x \cos y$$

Key concepts

Partial Derivatives

Partial derivatives play a pivotal role in multivariable calculus, particularly when analyzing the behavior of functions with more than one independent variable. When working with a function like \(f(x, y) = x^{2} \text{sin} y\), determining the rate at which \(f\) changes with respect to each variable individually is crucial. This rate of change is captured by taking partial derivatives.

For instance, when we compute \(f_x(x, y)\), it represents the slope of the surface created by \(f\) along the direction of the \(x\)-axis, while keeping \(y\) constant. Similarly, the derivative \(f_y(x, y)\) represents the slope along the \(y\)-axis, with \(x\) held constant. Understanding these derivatives helps visualize the shape and gradient of the function's graph in the spatial plane.

Knowing the concept of the first partial derivatives is essential before one can delve into second partial derivatives, which provide information about the curvature and concavity of the surface derived from the function.

Multivariable Calculus

Multivariable calculus extends the principles of single-variable calculus to functions of several variables. The function \(f(x, y) = x^{2} \text{sin} y\) from our exercise is an example of a multivariable function since it depends on two independent variables, \(x\) and \(y\). In the realm of multivariable calculus, partial derivatives are just the first step towards understanding more complex behaviors of such functions.

Second partial derivatives, such as \(f_{xx}(x, y)\) and \(f_{yy}(x, y)\), further illuminate how the function's graph bends in different directions. If one imagines the surface that such a function represents, second partial derivatives can indicate whether the surface has a hill, valley, or saddle point nearby a specific point.

Additionally, analyzing second partial derivatives is fundamental when performing optimization tasks, for solving problems regarding local maxima and minima—the peaks and troughs of the function's graph.

Chain Rule in Calculus

The chain rule is a fundamental theorem in calculus that allows us to differentiate composite functions—functions made up by chaining together other functions. In the context of multivariable calculus, the chain rule becomes even more powerful as it lets us differentiate functions with respect to multiple variables that are themselves functions of other variables.

For instance, if one needs to find the rate of change of \(f(x(t), y(t))\) with respect to time \(t\), where \(x\) and \(y\) are functions of \(t\), we would use the multivariable chain rule. It combines the derivatives of \(f\) with respect to \(x\) and \(y\), and the derivatives of \(x\) and \(y\) with respect to \(t\), giving us a full picture of how \(f\) changes as \(t\) progresses.

This is essential when dealing with physical problems where variables depend on time or other parameters, allowing us to track how changes propagate through a system. Understanding the chain rule is vital for students to effectively tackle problems involving partial derivatives and optimize functions defined by multivariable systems.