Value of $\displaystyle \lim_{x \to 0} g(x)$ Given Limit Statements

In summary, the value of $\displaystyle \lim_{x\to 0} g(x)$ must be equal to $4$ if the limit statement $\displaystyle \lim_{x\to 0} \left(\frac{4-g(x)}{x} \right)=1$ is to hold. This can be seen by considering the indeterminate form of the limit and finding that $x$ must be a factor of $4-g(x)$ for the limit to be equal to $1$. With this in mind, we can use the limit laws to show that $4-\lim_{x\to 0}g(x) = 0\cdot 1$, leading to $\lim_{x\to
  • #1
karush
Gold Member
MHB
3,269
5
$\textsf{find the value that $\displaystyle \lim_{x \to 0} g(x)$ must have if the
given limit statements hold.}$
$$\displaystyle \lim_{x \to 0} \left(\frac{4-g(x)}{x} \right)=1$$

OK the only answer I saw by observation was 2 but the book says it is 4
not sure how you get it with steps
 
Last edited:
Physics news on Phys.org
  • #2
karush said:
$\textsf{find the value that $\displaystyle \lim_{x \to 0} g(x)$ must have if the
given limit statements hold.}$
$$\displaystyle \lim_{x \to 0} \left(\frac{4-g(x)}{x} \right)=1$$

OK the only answer I saw by observation was 2 but the book says it is 4
not sure how you get it with steps

2 doesn't work; if $\displaystyle\lim_{x\to 0} g(x)=2$, then $\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x} \rightarrow \frac{2}{0}$, which is undefined.

For $\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x}=1$, $x$ must be a factor of $4-g(x)$. Suppose $4-g(x) = x f(x)$, where $g(x)$ and $f(x)$ both have limits as $x\to 0$. Then

\[\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x} = \lim_{x\to 0}f(x) = 1.\]

Furthermore, note that $\displaystyle\lim_{x\to 0}(4-g(x)) = \lim_{x\to 0}x f(x)$. Since $\displaystyle\lim_{x\to 0}f(x)=1$, we find that

\[\lim_{x\to 0}(4-g(x)) =\lim_{x\to 0}xf(x) \implies 4-\lim_{x\to 0}g(x) = 0\cdot 1 \implies \lim_{x\to 0}g(x) = 4.\]

I hope this makes sense!
 
  • #3
Alternatively, recall your work with indeterminate forms. What value must $g(x)$ take to obtain $\frac00$ in the limit statement?
 
  • #4
Chris L T521 said:
2 doesn't work; if $\displaystyle\lim_{x\to 0} g(x)=2$, then $\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x} \rightarrow \frac{2}{0}$, which is undefined.
For $\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x}=1$, $x$ must be a factor of $4-g(x)$. Suppose $4-g(x) = x f(x)$, where $g(x)$ and $f(x)$ both have limits as $x\to 0$. Then
\[\displaystyle\lim_{x\to 0}\frac{4-g(x)}{x} = \lim_{x\to 0}f(x) = 1.\]
Furthermore, note that $\displaystyle\lim_{x\to 0}(4-g(x)) = \lim_{x\to 0}x f(x)$. Since $\displaystyle\lim_{x\to 0}f(x)=1$, we find that
\[\lim_{x\to 0}(4-g(x)) =\lim_{x\to 0}xf(x) \implies 4-\lim_{x\to 0}g(x) = 0\cdot 1 \implies \lim_{x\to 0}g(x) = 4.\]
I hope this makes sense!

yes thank you for the expanded explanation the book was to short on the subject
 

FAQ: Value of $\displaystyle \lim_{x \to 0} g(x)$ Given Limit Statements

What is the definition of a limit statement?

A limit statement is a mathematical concept that describes the behavior of a function as its input approaches a certain value or point. It is denoted by the notation $\displaystyle \lim_{x \to a} f(x)$, where $a$ is the point of approach.

How is the value of $\displaystyle \lim_{x \to a} f(x)$ determined?

The value of $\displaystyle \lim_{x \to a} f(x)$ is determined by evaluating the function at values of $x$ that are very close to the point $a$. This can be done by using a table, graph, or algebraic techniques such as factoring or simplifying.

What does it mean for a limit statement to exist?

A limit statement exists if the value of $\displaystyle \lim_{x \to a} f(x)$ is the same regardless of the approach from either side of $a$. This means that the function approaches the same value as $x$ gets closer and closer to $a$.

How does the value of a limit statement relate to the actual value of the function at the given point?

The value of $\displaystyle \lim_{x \to a} f(x)$ does not necessarily equal the actual value of the function at $a$. It only describes the behavior of the function as it gets closer to $a$. The actual value of the function at $a$ may or may not exist or be equal to the limit statement.

What are the different types of limit statements?

There are three types of limit statements: finite, infinite, and undefined. A finite limit statement is when the function approaches a specific value as $x$ approaches a certain point. An infinite limit statement is when the function approaches positive or negative infinity as $x$ approaches a certain point. An undefined limit statement is when the function does not approach a specific value or infinity as $x$ approaches a certain point.

Similar threads

Replies
4
Views
2K
Replies
16
Views
3K
Replies
3
Views
1K
Replies
9
Views
2K
Replies
4
Views
1K
Replies
1
Views
2K
Back
Top