When is a limit considered to exist?

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In summary, the value of k that makes the limit of (x^2-kx+4)/(x-1) exist is 5. By setting the numerator equal to zero, it ensures that x-1 is a factor and cancels with the denominator, preventing the fraction from becoming infinite near x=1. Therefore, the solution is not assuming a finite limit, but rather stating a fact that a finite limit exists when the top is set to zero.
  • #1
Jimmy25
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Homework Statement



Find a value of k such that the limit exists

lim (x2-kx+4)/(x-1)
x->1

The Attempt at a Solution



In the solution they set the top equal to zero finding that k is equal to 5. Why do you assume the numerator must equal zero if the denominator equals zero? Is this the only case in which the limit exists? If so why?
 
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  • #2
If the top equals anything else, the fraction (x^2-kx+4)/(x-1) would blow up near x=1 because the denominator approaches 0. Setting the top to 0 ensures that x-1 is a factor of the numerator. The x-1 on the top would then cancel with the denominator, preventing the fraction from becoming infinity near x=1.
 
  • #3
So in this case, they are assuming that an infinite limit does not exist?
 
  • #4
They are implicitly saying that a finite limit exists.
 
  • #5
Jimmy25 said:
So in this case, they are assuming that an infinite limit does not exist?
I would not use the word "assuming" here- it is a fact.

Saying that a limit "is infinity" or "is negative infinity" is just saying that the limit does not exist for a specific reason.
 

FAQ: When is a limit considered to exist?

What is the definition of the limit?

The limit of a function is the value that a function approaches as the input approaches a specific value. This value can be either a number or infinity.

How do you determine if the limit exists?

The limit exists if the left-hand limit (approaching the value from the left) and the right-hand limit (approaching the value from the right) are equal. If they are not equal, the limit does not exist.

Can the limit exist at the point where the function is undefined?

No, the limit cannot exist at a point where the function is undefined. This is because the function does not have a value at that point and therefore the limit cannot be determined.

What are some common ways to evaluate limits?

There are several methods for evaluating limits, including direct substitution, factoring, and using algebraic manipulation. Other methods include using limits laws, L'Hopital's rule, and the squeeze theorem.

What is the relationship between continuity and the existence of a limit?

A function is continuous if the limit at a point exists and is equal to the function value at that point. Therefore, the existence of a limit is necessary for a function to be continuous. However, the existence of a limit does not always guarantee continuity.

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