Use Continuity to Evaluate the Limit Lim Xa 4 SinX Si

Learning Objectives

• 4.2.1 Calculate the limit of a function of two variables.
• 4.2.2 Learn how a function of two variables can approach different values at a boundary point, depending on the path of approach.
• 4.2.3 State the conditions for continuity of a function of two variables.
• 4.2.4 Verify the continuity of a function of two variables at a point.
• 4.2.5 Calculate the limit of a function of three or more variables and verify the continuity of the function at a point.

We have now examined functions of more than one variable and seen how to graph them. In this section, we see how to take the limit of a function of more than one variable, and what it means for a function of more than one variable to be continuous at a point in its domain. It turns out these concepts have aspects that just don't occur with functions of one variable.

Limit of a Function of Two Variables

Recall from The Limit of a Function the definition of a limit of a function of one variable:

Let $f\left(x\right)$ be defined for all $x\ne a$ in an open interval containing $a.$ Let $L$ be a real number. Then

$$\underset{x\to a}{\text{lim}}f(x)=L$$

if for every $\epsilon >0,$ there exists a $\delta >0,$ such that if $0<\left|x-a\right|<\delta$ for all $x$ in the domain of $f,$ then

$$\left|f\left(x\right)-L\right|<\epsilon .$$

Before we can adapt this definition to define a limit of a function of two variables, we first need to see how to extend the idea of an open interval in one variable to an open interval in two variables.

Definition

Consider a point $(a,b)\in {ℝ}^2.$ A $\delta$ disk centered at point $(a,b)$ is defined to be an open disk of radius $\delta$ centered at point $\left(a,b\right)$ —that is,

$$\{(x,y)\in {ℝ}^2|{(x-a)}^2+{(y-b)}^2<{\delta }^2\}$$

as shown in the following graph.

Figure 4.14 A $\delta$ disk centered around the point $\left(2,1\right).$

The idea of a $\delta$ disk appears in the definition of the limit of a function of two variables. If $\delta$ is small, then all the points $\left(x,y\right)$ in the $\delta$ disk are close to $\left(a,b\right).$ This is completely analogous to $x$ being close to $a$ in the definition of a limit of a function of one variable. In one dimension, we express this restriction as

$$a-\delta <x<a+\delta .$$

In more than one dimension, we use a $\delta$ disk.

Definition

Let $f$ be a function of two variables, $x$ and $y.$ The limit of $f\left(x,y\right)$ as $\left(x,y\right)$ approaches $\left(a,b\right)$ is $L,$ written

$$\underset{\left(x,y\right)\to \left(a,b\right)}{\text{lim}}f(x,y)=L$$

if for each $\epsilon >0$ there exists a small enough $\delta >0$ such that for all points $\left(x,y\right)$ in a $\delta$ disk around $\left(a,b\right),$ except possibly for $\left(a,b\right)$ itself, the value of $f\left(x,y\right)$ is no more than $\epsilon$ away from $L$ (Figure 4.15). Using symbols, we write the following: For any $\epsilon >0,$ there exists a number $\delta >0$ such that

$$\left|f\left(x,y\right)-L\right|<\epsilon \text{whenever}0<\sqrt{{\left(x-a\right)}^2+{\left(y-b\right)}^2}<\delta .$$

Figure 4.15 The limit of a function involving two variables requires that $f\left(x,y\right)$ be within $\epsilon$ of $L$ whenever $(x,y)$ is within $\delta$ of $(a,b).$ The smaller the value of $\epsilon ,$ the smaller the value of $\delta .$

Proving that a limit exists using the definition of a limit of a function of two variables can be challenging. Instead, we use the following theorem, which gives us shortcuts to finding limits. The formulas in this theorem are an extension of the formulas in the limit laws theorem in The Limit Laws.

Theorem 4.1

Limit laws for functions of two variables

Let $f\left(x,y\right)$ and $g\left(x,y\right)$ be defined for all $\left(x,y\right)\ne \left(a,b\right)$ in a neighborhood around $\left(a,b\right),$ and assume the neighborhood is contained completely inside the domain of $f.$ Assume that $L$ and $M$ are real numbers such that $\underset{(x,y)\to (a,b)}{\text{lim}}f(x,y)=L$ and $\underset{(x,y)\to (a,b)}{\text{lim}}g(x,y)=M,$ and let $c$ be a constant. Then each of the following statements holds:

Constant Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}c=c$$

(4.2)

Identity Laws:

$$\underset{(x,y)\to (a,b)}{\text{lim}}x=a$$

(4.3)

$$\underset{(x,y)\to (a,b)}{\text{lim}}y=b$$

(4.4)

Sum Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\left(f\left(x,y\right)+g\left(x,y\right)\right)=L+M$$

(4.5)

Difference Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\left(f\left(x,y\right)-g\left(x,y\right)\right)=L-M$$

(4.6)

Constant Multiple Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\left(cf\left(x,y\right)\right)=cL$$

(4.7)

Product Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\left(f\left(x,y\right)g\left(x,y\right)\right)=LM$$

(4.8)

Quotient Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\frac{f\left(x,y\right)}{g\left(x,y\right)}=\frac{L}{M}\text{for}M\ne 0$$

(4.9)

Power Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}{\left(f\left(x,y\right)\right)}^n=L^n$$

(4.10)

for any positive integer $n.$

Root Law:

$$\underset{(x,y)\to (a,b)}{\text{lim}}\sqrt[n]{f\left(x,y\right)}=\sqrt[n]{L}$$

(4.11)

for all $L$ if $n$ is odd and positive, and for $L\ge 0$ if $n$ is even and positive provided that $f(x,y)\ge 0$ for all $(x,y)\ne (a,b)$ in neighborhood of $(a,b)$ .

The proofs of these properties are similar to those for the limits of functions of one variable. We can apply these laws to finding limits of various functions.

Example 4.8

Finding the Limit of a Function of Two Variables

Find each of the following limits:

1. $\underset{\left(x,y\right)\to \left(2,\mathrm{-1}\right)}{\text{lim}}\left(x^2-2xy+3y^2-4x+3y-6\right)$
2. $\underset{\left(x,y\right)\to \left(2,\mathrm{-1}\right)}{\text{lim}}\frac{2x+3y}{4x-3y}$

Checkpoint 4.6

Evaluate the following limit:

$$\underset{(x,y)\to (5,\mathrm{-2})}{\text{lim}}\sqrt[3]{\frac{x^2-y}{y^2+x-1}}.$$

Since we are taking the limit of a function of two variables, the point $(a,b)$ is in ${ℝ}^2,$ and it is possible to approach this point from an infinite number of directions. Sometimes when calculating a limit, the answer varies depending on the path taken toward $(a,b).$ If this is the case, then the limit fails to exist. In other words, the limit must be unique, regardless of path taken.

Example 4.9

Limits That Fail to Exist

Show that neither of the following limits exist:

1. $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{2xy}{3x^2+y^2}$
2. $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{4x{y}^2}{x^2+3y^4}$

Checkpoint 4.7

Show that

$$\underset{\left(x,y\right)\to \left(2,1\right)}{\text{lim}}\frac{\left(x-2\right)\left(y-1\right)}{{\left(x-2\right)}^2+{\left(y-1\right)}^2}$$

does not exist.

Interior Points and Boundary Points

To study continuity and differentiability of a function of two or more variables, we first need to learn some new terminology.

Definition

Let S be a subset of ${ℝ}^2$ (Figure 4.17).

1. A point $P_0$ is called an interior point of $S$ if there is a $\delta$ disk centered around $P_0$ contained completely in $S.$
2. A point $P_0$ is called a boundary point of $S$ if every $\delta$ disk centered around $P_0$ contains points both inside and outside $S.$

Figure 4.17 In the set $S$ shown, $\left(\mathrm{-1},1\right)$ is an interior point and $\left(2,3\right)$ is a boundary point.

Definition

Let S be a subset of ${ℝ}^2$ (Figure 4.17).

1. $S$ is called an open set if every point of $S$ is an interior point.
2. $S$ is called a closed set if it contains all its boundary points.

An example of an open set is a $\delta$ disk. If we include the boundary of the disk, then it becomes a closed set. A set that contains some, but not all, of its boundary points is neither open nor closed. For example if we include half the boundary of a $\delta$ disk but not the other half, then the set is neither open nor closed.

Definition

Let S be a subset of ${ℝ}^2$ (Figure 4.17).

1. An open set $S$ is a connected set if it cannot be represented as the union of two or more disjoint, nonempty open subsets.
2. A set $S$ is a region if it is open, connected, and nonempty.

The definition of a limit of a function of two variables requires the $\delta$ disk to be contained inside the domain of the function. However, if we wish to find the limit of a function at a boundary point of the domain, the $\delta \text{disk}$ is not contained inside the domain. By definition, some of the points of the $\delta \text{disk}$ are inside the domain and some are outside. Therefore, we need only consider points that are inside both the $\delta$ disk and the domain of the function. This leads to the definition of the limit of a function at a boundary point.

Definition

Let $f$ be a function of two variables, $x$ and $y,$ and suppose $\left(a,b\right)$ is on the boundary of the domain of $f.$ Then, the limit of $f\left(x,y\right)$ as $\left(x,y\right)$ approaches $\left(a,b\right)$ is $L,$ written

$$\underset{\left(x,y\right)\to \left(a,b\right)}{\text{lim}}f\left(x,y\right)=L,$$

if for any $\epsilon >0,$ there exists a number $\delta >0$ such that for any point $\left(x,y\right)$ inside the domain of $f$ and within a suitably small distance positive $\delta$ of $\left(a,b\right),$ the value of $f\left(x,y\right)$ is no more than $\epsilon$ away from $L$ (Figure 4.15). Using symbols, we can write: For any $\epsilon >0,$ there exists a number $\delta >0$ such that

$$\left|f\left(x,y\right)-L\right|<\epsilon \text{whenever}0<\sqrt{{\left(x-a\right)}^2+{\left(y-b\right)}^2}<\delta .$$

Example 4.10

Limit of a Function at a Boundary Point

Prove $\underset{\left(x,y\right)\to \left(4,3\right)}{\text{lim}}\sqrt{25-x^2-y^2}=0.$

Checkpoint 4.8

Evaluate the following limit:

$$\underset{\left(x,y\right)\to \left(5,\mathrm{-2}\right)}{\text{lim}}\sqrt{29-x^2-y^2}.$$

Continuity of Functions of Two Variables

In Continuity, we defined the continuity of a function of one variable and saw how it relied on the limit of a function of one variable. In particular, three conditions are necessary for $f\left(x\right)$ to be continuous at point $x=a\text{:}$

1. $f\left(a\right)$ exists.
2. $\underset{x\to a}{\text{lim}}f\left(x\right)$ exists.
3. $\underset{x\to a}{\text{lim}}f\left(x\right)=f\left(a\right).$

These three conditions are necessary for continuity of a function of two variables as well.

Definition

A function $f(x,y)$ is continuous at a point $\left(a,b\right)$ in its domain if the following conditions are satisfied:

1. $f\left(a,b\right)$ exists.
2. $\underset{\left(x,y\right)\to \left(a,b\right)}{\text{lim}}f\left(x,y\right)$ exists.
3. $\underset{\left(x,y\right)\to \left(a,b\right)}{\text{lim}}f\left(x,y\right)=f\left(a,b\right).$

Example 4.11

Demonstrating Continuity for a Function of Two Variables

Show that the function $f(x,y)=\frac{3x+2y}{x+y+1}$ is continuous at point $\left(5,\mathrm{-3}\right).$

Checkpoint 4.9

Show that the function $f\left(x,y\right)=\sqrt{26-2x^2-y^2}$ is continuous at point $\left(2,\mathrm{-3}\right).$

Continuity of a function of any number of variables can also be defined in terms of delta and epsilon. A function of two variables is continuous at a point $\left(x_0,y_0\right)$ in its domain if for every $\epsilon >0$ there exists a $\delta >0$ such that, whenever $\sqrt{{\left(x-x_0\right)}^2+{\left(y-y_0\right)}^2}<\delta$ it is true, $\left|f\left(x,y\right)-f\left(a,b\right)\right|<\epsilon .$ This definition can be combined with the formal definition (that is, the epsilon–delta definition) of continuity of a function of one variable to prove the following theorems:

Theorem 4.2

The Sum of Continuous Functions Is Continuous

If $f\left(x,y\right)$ is continuous at $\left(x_0,y_0\right),$ and $g\left(x,y\right)$ is continuous at $\left(x_0,y_0\right),$ then $f\left(x,y\right)+g\left(x,y\right)$ is continuous at $\left(x_0,y_0\right).$

Theorem 4.3

The Product of Continuous Functions Is Continuous

If $g\left(x\right)$ is continuous at $x_0$ and $h\left(y\right)$ is continuous at $y_0,$ then $f\left(x,y\right)=g\left(x\right)h\left(y\right)$ is continuous at $\left(x_0,y_0\right).$

Theorem 4.4

The Composition of Continuous Functions Is Continuous

Let $g$ be a function of two variables from a domain $D\subseteq {ℝ}^2$ to a range $R\subseteq ℝ.$ Suppose $g$ is continuous at some point $\left(x_0,y_0\right)\in D$ and define $z_0=g\left(x_0,y_0\right).$ Let $f$ be a function that maps $ℝ$ to $ℝ$ such that $z_0$ is in the domain of $f.$ Last, assume $f$ is continuous at $z_0.$ Then $f\circ g$ is continuous at $\left(x_0,y_0\right)$ as shown in the following figure.

Figure 4.20 The composition of two continuous functions is continuous.

Let's now use the previous theorems to show continuity of functions in the following examples.

Example 4.12

More Examples of Continuity of a Function of Two Variables

Show that the functions $f\left(x,y\right)=4x^3{y}^2$ and $g\left(x,y\right)=\text{cos}\left(4x^3{y}^2\right)$ are continuous everywhere.

Checkpoint 4.10

Show that the functions $f\left(x,y\right)=2x^2{y}^3+3$ and $g\left(x,y\right)={\left(2x^2{y}^3+3\right)}^4$ are continuous everywhere.

Functions of Three or More Variables

The limit of a function of three or more variables occurs readily in applications. For example, suppose we have a function $f\left(x,y,z\right)$ that gives the temperature at a physical location $\left(x,y,z\right)$ in three dimensions. Or perhaps a function $g\left(x,y,z,t\right)$ can indicate air pressure at a location $\left(x,y,z\right)$ at time $t.$ How can we take a limit at a point in ${ℝ}^3?$ What does it mean to be continuous at a point in four dimensions?

The answers to these questions rely on extending the concept of a $\delta$ disk into more than two dimensions. Then, the ideas of the limit of a function of three or more variables and the continuity of a function of three or more variables are very similar to the definitions given earlier for a function of two variables.

Definition

Let $\left(x_0,y_0,z_0\right)$ be a point in ${ℝ}^3.$ Then, a $\delta$ ball in three dimensions consists of all points in ${ℝ}^3$ lying at a distance of less than $\delta$ from $\left(x_0,y_0,z_0\right)$ —that is,

$$\left\{\left(x,y,z\right)\in {ℝ}^3|\sqrt{{\left(x-x_0\right)}^2+{\left(y-y_0\right)}^2+{\left(z-z_0\right)}^2}<\delta \right\}.$$

To define a $\delta$ ball in higher dimensions, add additional terms under the radical to correspond to each additional dimension. For example, given a point $P=\left(w_0,x_0,y_0,z_0\right)$ in ${ℝ}^4,$ a $\delta$ ball around $P$ can be described by

$$\left\{\left(w,x,y,z\right)\in {ℝ}^4|\sqrt{{\left(w-w_0\right)}^2+{\left(x-x_0\right)}^2+{\left(y-y_0\right)}^2+{\left(z-z_0\right)}^2}<\delta \right\}.$$

To show that a limit of a function of three variables exists at a point $\left(x_0,y_0,z_0\right),$ it suffices to show that for any point in a $\delta$ ball centered at $\left(x_0,y_0,z_0\right),$ the value of the function at that point is arbitrarily close to a fixed value (the limit value). All the limit laws for functions of two variables hold for functions of more than two variables as well.

Example 4.13

Finding the Limit of a Function of Three Variables

Find $\underset{\left(x,y,z\right)\to \left(4,1,\mathrm{-3}\right)}{\text{lim}}\frac{x^{2}y-3z}{2x+5y-z}.$

Checkpoint 4.11

Find $\underset{\left(x,y,z\right)\to \left(4,\mathrm{-1},3\right)}{\text{lim}}\sqrt{13-x^2-2y^2+z^2}.$

Section 4.2 Exercises

For the following exercises, find the limit of the function.

60 .

$\underset{(x,y)\to (1,2)}{\text{lim}}x$

61.

$\underset{(x,y)\to (1,2)}{\text{lim}}\frac{5x^{2}y}{x^2+y^2}$

62 .

Show that the limit $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{5x^{2}y}{x^2+y^2}$ exists and is the same along the paths: $y\text{-axis}$ and $x\text{-axis,}$ and along $y=x.$

For the following exercises, evaluate the limits at the indicated values of $x\text{and}y.$ If the limit does not exist, state this and explain why the limit does not exist.

63.

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{4x^2+10y^2+4}{4x^2-10y^2+6}$

64 .

$\underset{(x,y)\to (11,13)}{\text{lim}}\sqrt{\frac{1}{xy}}$

65.

$\underset{(x,y)\to (0,1)}{\text{lim}}\frac{y^2\text{sin}x}{x}$

66 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\text{sin}\left(\frac{x^8+y^7}{x-y+10}\right)$

67.

$\underset{(x,y)\to (\pi \text{/}4,1)}{\text{lim}}\frac{y\text{tan}x}{y+1}$

68 .

$\underset{(x,y)\to (0,\pi \text{/}4)}{\text{lim}}\frac{\text{sec}x+2}{3x-\text{tan}y}$

69.

$\underset{(x,y)\to (2,5)}{\text{lim}}\left(\frac{1}{x}-\frac{5}{y}\right)$

70 .

$\underset{(x,y)\to (4,4)}{\text{lim}}x\text{ln}y$

71.

$\underset{(x,y)\to (4,4)}{\text{lim}}{e}^{\text{−}{x}^2-y^2}$

72 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\sqrt{9-x^2-y^2}$

73.

$\underset{(x,y)\to (1,2)}{\text{lim}}\left(x^2{y}^3-x^3{y}^2+3x+2y\right)$

74 .

$\underset{(x,y)\to (\pi ,\pi )}{\text{lim}}x\text{sin}\left(\frac{x+y}{4}\right)$

75.

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{xy+1}{x^2+y^2+1}$

76 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^2+y^2}{\sqrt{x^2+y^2+1}-1}$

77.

$\underset{(x,y)\to (0,0)}{\text{lim}}\text{ln}\left(x^2+y^2\right)$

For the following exercises, complete the statement.

78 .

A point $\left(x_0,y_0\right)$ in a plane region $R$ is an interior point of $R$ if _________________.

79.

A point $\left(x_0,y_0\right)$ in a plane region $R$ is called a boundary point of $R$ if ___________.

For the following exercises, use algebraic techniques to evaluate the limit.

80 .

$\underset{(x,y)\to (2,1)}{\text{lim}}\frac{x-y-1}{\sqrt{x-y}-1}$

81.

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^4-4y^4}{x^2+2y^2}$

82 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^3-y^3}{x-y}$

83.

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^2-xy}{\sqrt{x}-\sqrt{y}}$

For the following exercises, evaluate the limits of the functions of three variables.

84 .

$\underset{(x,y,z)\to (1,2,3)}{\text{lim}}\frac{x{z}^2-y^{2}z}{xyz-1}$

85.

$\underset{(x,y,z)\to (0,0,0)}{\text{lim}}\frac{x^2-y^2-z^2}{x^2+y^2-z^2}$

For the following exercises, evaluate the limit of the function by determining the value the function approaches along the indicated paths. If the limit does not exist, explain why not.

86 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{xy+y^3}{x^2+y^2}$

1. Along the $x\text{-axis}$ $(y=0)$
2. Along the $y\text{-axis}$ $(x=0)$
3. Along the path $y=2x$

87.

Evaluate $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{xy+y^3}{x^2+y^2}$ using the results of previous problem.

88 .

$\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^{2}y}{x^4+y^2}$

1. Along the x-axis $(y=0)$
2. Along the y-axis $(x=0)$
3. Along the path $y=x^2$

89.

Evaluate $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{x^{2}y}{x^4+y^2}$ using the results of previous problem.

Discuss the continuity of the following functions. Find the largest region in the $xy\text{-plane}$ in which the following functions are continuous.

90 .

$f(x,y)=\text{sin}(xy)$

91.

$f(x,y)=\text{ln}(x+y)$

92 .

$f(x,y)=e^{3xy}$

93.

$f(x,y)=\frac{1}{xy}$

For the following exercises, determine the region in which the function is continuous. Explain your answer.

94 .

$f(x,y)=\frac{x^{2}y}{x^2+y^2}$

95.

$f(x,y)=\left\{\begin{array}{cc}\frac{x^{2}y}{x^2+y^2}\hfill & \text{if}(x,y)\ne (0,0)\hfill \\ 0\hfill & \text{if}(x,y)=(0,0)\hfill \end{array}\right\}$

(Hint: Show that the function approaches different values along two different paths.)

96 .

$f(x,y)=\frac{\text{sin}(x^2+y^2)}{x^2+y^2}$

97.

Determine whether $g(x,y)=\frac{x^2-y^2}{x^2+y^2}$ is continuous at $\left(0,0\right).$

98 .

Create a plot using graphing software to determine where the limit does not exist. Find where in the coordinate plane $f(x,y)=\frac{1}{x^2-y}$ is continuous.

99.

Determine the region of the $xy\text{-plane}$ in which the function $g(x,y)=\text{arctan}\left(\frac{x{y}^2}{x+y}\right)$ is continuous. Use technology to support your conclusion.

100 .

Determine the region of the $xy\text{-plane}$ in which $f(x,y)=\text{ln}(x^2+y^2-1)$ is continuous. Use technology to support your conclusion. (Hint: Choose the range of values for $x\text{and}y$ carefully!)

101.

At what points in space is $g(x,y,z)=x^2+y^2-2z^2$ continuous?

102 .

At what points in space is $g(x,y,z)=\frac{1}{x^2+z^2-1}$ continuous?

103.

Show that $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{1}{x^2+y^2}$ does not exist at $\left(0,0\right)$ by plotting the graph of the function.

104 .

[T] Evaluate $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{\text{−}x{y}^2}{x^2+y^4}$ by plotting the function using a CAS. Determine analytically the limit along the path $x=y^2.$

105.

[T]

1. Use a CAS to draw a contour map of $z=\sqrt{9-x^2-y^2}.$
2. What is the name of the geometric shape of the level curves?
3. Give the general equation of the level curves.
4. What is the maximum value of $z?$
5. What is the domain of the function?
6. What is the range of the function?

106 .

True or False: If we evaluate $\underset{(x,y)\to (0,0)}{\text{lim}}f(x)$ along several paths and each time the limit is $1,$ we can conclude that $\underset{(x,y)\to (0,0)}{\text{lim}}f(x)=1.$

107.

Use polar coordinates to find $\underset{(x,y)\to (0,0)}{\text{lim}}\frac{\text{sin}\sqrt{x^2+y^2}}{\sqrt{x^2+y^2}}.$ You can also find the limit using L'Hôpital's rule.

108 .

Use polar coordinates to find $\underset{(x,y)\to (0,0)}{\text{lim}}\text{cos}\left(x^2+y^2\right).$

109.

Discuss the continuity of $f(g(x,y))$ where $f(t)=1\text{/}t$ and $g(x,y)=2x-5y.$

110 .

Given $f(x,y)=x^2-4y,$ find $\underset{h\to 0}{\text{lim}}\frac{f(x+h,y)-f(x,y)}{h}.$

111.

Given $f(x,y)=x^2-4y,$ find $\underset{h\to 0}{\text{lim}}\frac{f(1+h,y)-f(1,y)}{h}.$

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Source: https://openstax.org/books/calculus-volume-3/pages/4-2-limits-and-continuity