Problem

Use the two-path test to prove that the following limit does not exist. \[ \lim _{(x, y) \rightarrow(0,0)} \frac{x+2 y}{x-2 y} \] What value does $f(x, y)=\frac{x+2 y}{x-2 y}$ approach as $(x, y)$ approaches $(0,0)$ along the $x$-axis? Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. $f(x, y)$ approaches $\square$. (Simplify your answer.) B. $f(x, y)$ has no limit and does not approach $\infty$ or $-\infty$ as $(x, y)$ approaches $(0,0)$ along the $x$-axis. What value does $f(x, y)=\frac{x+2 y}{x-2 y}$ approach as $(x, y)$ approaches $(0,0)$ along the $y$-axis? Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. $f(x, y)$ approaches (Simplify your answer.) B. $f(x, y)$ has no limit and does not approach $\infty$ or $-\infty$ as $(x, y)$ approaches $(0,0)$ along the $y$-axis. Why does the given limit not exist? A. The limit does not exist because as $(x, y)$ approaches $(0,0)$, the denominator approaches 0 . B. As $(x, y)$ approaches $(0,0)$ along different paths, $f(x, y)$ approaches two different values. C. As $(x, y)$ approaches $(0,0)$ along different paths, $f(x, y)$ always approaches the same value. D. As $(x, y)$ approaches $(0,0)$ along different paths, $f(x, y)$ does not always approach a finite value.

Solution

Step 1 :Consider the function \(f(x, y)=\frac{x+2 y}{x-2 y}\).

Step 2 :We first find the limit of the function as \((x, y)\) approaches \((0,0)\) along the x-axis. This means we set \(y=0\) in the function.

Step 3 :Substituting \(y=0\) into the function, we get \(f(x, 0)=\frac{x+2(0)}{x-2(0)}=\frac{x}{x}=1\).

Step 4 :So, the limit of the function as \((x, y)\) approaches \((0,0)\) along the x-axis is \(\boxed{1}\).

Step 5 :Next, we find the limit of the function as \((x, y)\) approaches \((0,0)\) along the y-axis. This means we set \(x=0\) in the function.

Step 6 :Substituting \(x=0\) into the function, we get \(f(0, y)=\frac{0+2y}{0-2y}=\frac{2y}{-2y}=-1\).

Step 7 :So, the limit of the function as \((x, y)\) approaches \((0,0)\) along the y-axis is \(\boxed{-1}\).

Step 8 :Since the limits along the x-axis and y-axis are different, the limit of the function as \((x, y)\) approaches \((0,0)\) does not exist.

Step 9 :This is because the definition of the limit requires the function to approach the same value regardless of the path taken to \((0,0)\).

Step 10 :So, the limit does not exist because as \((x, y)\) approaches \((0,0)\) along different paths, \(f(x, y)\) approaches two different values.

Step 11 :Final Answer: The limit of the function as \((x, y)\) approaches \((0,0)\) along the x-axis is \(\boxed{1}\). The limit of the function as \((x, y)\) approaches \((0,0)\) along the y-axis is \(\boxed{-1}\). The limit does not exist because as \((x, y)\) approaches \((0,0)\) along different paths, \(f(x, y)\) approaches two different values.

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Source: https://solvelyapp.com/problems/17241/

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