Why does using the substitution u=x^2 help evaluate the limit as x approaches 0?

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In summary, the conversation revolves around solving a limit using the factoring method which does not work. The correct method to solve the limit is to use the substitution u=x^2 and then applying the Hospital rule three times. The final answer is 4/5.
  • #1
JustinLiang
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Homework Statement


http://www.prep101.com/files/Math100PracticeExam.pdf

Question 1m

The Attempt at a Solution


I tried to do this by using
lim sinx/x = 1
x->0

Factoring out x^2 from each one and I get infinity.
Why does this method not work?

The answer says it's 4/5.

I even tried plugging in 0.001 in my calculator and I get infinity :S.
 
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  • #2
Is that sin(2x^2), sin^2(2x), sin(2)x^2 ?
 
  • #3
QuarkCharmer said:
Is that sin(2x^2), sin^2(2x), sin(2)x^2 ?

Or is it 3sin(2x^2-6x^2+4x^6)? The question isn't very grammatical. And however I rearrange it I still can't figure out how to pull 4/5 as a limit out.
 
  • #4
Dick said:
Or is it 3sin(2x^2-6x^2+4x^6)? The question isn't very grammatical. And however I rearrange it I still can't figure out how to pull 4/5 as a limit out.

It is 3sin(2x^2), you get 4/5, this limit is INSANE :P
 
  • #5
It helps to use the substitution u=x2 to get
[tex]\lim_{u \to 0}\frac{3 \sin 2u - 6u + 4u^3}{u^5}[/tex]Then three applications of the Hospital rule gets you to a limit you can evaluate.
 
  • #6
vela said:
It helps to use the substitution u=x2 to get
[tex]\lim_{u \to 0}\frac{3 \sin 2u - 6u + 4u^3}{u^5}[/tex]Then three applications of the Hospital rule gets you to a limit you can evaluate.

Ah, indeed it does.
 

1. What does it mean for x to approach 0?

When we say that x is approaching 0, it means that the value of x is getting closer and closer to 0, but is not actually equal to 0.

2. Why is the limit as x approaches 0 important?

The limit as x approaches 0 is important because it helps us understand the behavior of a function near the point x=0. It can also help us determine the continuity and differentiability of a function at x=0.

3. How do you find the limit as x approaches 0?

To find the limit as x approaches 0, we substitute 0 into the function and simplify. If the resulting value is undefined or infinite, we use algebraic techniques or L'Hopital's rule to evaluate the limit.

4. What is the difference between a one-sided limit and a two-sided limit?

A one-sided limit only considers the values of the function as x approaches 0 from one direction (either the left or the right). A two-sided limit takes into account the values of the function as x approaches 0 from both the left and the right.

5. Can the limit as x approaches 0 exist even if the function is not defined at x=0?

Yes, it is possible for the limit as x approaches 0 to exist even if the function is not defined at x=0. This is because the limit is concerned with the behavior of the function near x=0, not necessarily at x=0.

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