Limit of a Radical Expression without L'Hospital's Rule

In summary, after multiplying by the conjugate of 2-sqrt(x), you can plug in the next conjugate and it will just get messy.
  • #1
phrox
40
1

Homework Statement


limx->4 (sqrt(5-x)-1) / (2-sqrt(x))

NOT ALLOWED TO USE L'HOSPITALS.


Homework Equations





The Attempt at a Solution




I tried using conjugate of both top and bottom and I couldn't get it to work, but maybe I've done it wrong?.. I'm not allowed to use L'Hospital's rules so how can I solve this?
 
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  • #2
can you think of something you can multiply top and bottom by to get rid of the radical below the line? Does that give you any further ideas?
 
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  • #3
just by x? Because if I multiply by (2+sqrt(x)) it just got long and tedius, thought there was an easier way.
 
  • #4
We can't tell if you made a mistake in using the conjugate unless you post your work.
 
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  • #5
When I multiplied by the conjugate I ended up with 4-x on the denominator. Subbing in 4 would make the limit impossible.
 
  • #6
Multiplying both numerator and denominator by [itex]2+\sqrt{4}[/itex] will give an "[itex]x- 4[/itex]" in the denominator but multiplying both numerator and denominator by [itex]\sqrt{5- x}+ 1[/itex] gives "[itex](5- x)- 1= -(x- 4)[/itex]" in the numerator. What can you do now?
 
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  • #7
The only thing I see you can do with that is square it I guess?I'm not sure... Subbing in 4 will just make it 0.
 
  • #8
phrox said:
The only thing I see you can do with that is square it I guess?I'm not sure... Subbing in 4 will just make it 0.
No. I think what HallsofIvy is trying to tell you is that you have to multiply by the conjugate twice. See my post in this thread.
 
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  • #9
By rationalize do you mean multiply the conjugate? I don't see how you can rationalize (multiply top and bottom by the root), but you say you only do it to the top and then only do it to the bottom. I don't think you can do that. Also if I multiply the top conjugate and then bottom conjugate it gets REALLY messy and it can't be right.

(This is all after reading your post)
 
  • #10
phrox said:
By rationalize do you mean multiply the conjugate? I don't see how you can rationalize (multiply top and bottom by the root), but you say you only do it to the top and then only do it to the bottom. I don't think you can do that. Also if I multiply the top conjugate and then bottom conjugate it gets REALLY messy and it can't be right.

(This is all after reading your post)
You are not posting your working, so I have to guess. It sounds like you went through the full multiplication top and bottom each time. Don't do that. Multiply top and bottom by that expression which removes the radical from the denominator, but in the numerator just leave it as a product of two radical expressions. don't multiply that out. Now do the same for the radical expression which is the original numerator - multiply top and bottom by its radical 'conjugate'. You should now have an expression which is a product of two terms in the numerator (one being the conjugate of the original denominator, and the other not involving a radical) and similarly two terms in the denominator.
Now look at the two non-radical terms. Do you see any cancellation?
 
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  • #11
If you say don't multiply the 2 radical expressions out, am I leaving the (sqrt(5-x)-1)(2+sqrt(x)) in the top, then multiplying the top and bottom by sqrt(5-x)+1? So in the end I'd be multiplying 3 different expressions in the top:
(sqrt(5-x)-1)(2+sqrt(x))(sqrt(5-x)+1).

Here is what I get after multiplying by the conjugate of 2-sqrt(x). Then I plugged in the next conjugate and it just seems like it would get too messy.
http://i1301.photobucket.com/albums/ag115/phrox1/20130922_123901_zps9183e7f8.jpg
I have to be missing something here, it's not clicking in my head for some reason.
 
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  • #12
nevermind I understand now, thanks
 

Related to Limit of a Radical Expression without L'Hospital's Rule

1. What is a radical in a limit?

A radical in a limit is an expression that contains a root symbol, such as √x, and is used to represent a number or variable that is being taken to a certain power in a limit calculation.

2. How do you simplify a radical in a limit?

To simplify a radical in a limit, you can use the rules of exponents to rewrite the expression in a simpler form. This may involve factoring the number inside the radical or using the power rule to move the exponent outside of the radical.

3. What is the difference between a radical and a rational exponent in a limit?

A radical is a symbol that represents a root, while a rational exponent is a number that represents the power to which a base is raised. In a limit, both can be used interchangeably to represent the same value.

4. How do you solve a limit with two radicals?

To solve a limit with two radicals, you can use the properties of limits to break the expression into smaller limits and then evaluate each one individually. You can also use substitution or algebraic manipulation to simplify the expression before taking the limit.

5. Can you have a limit with two radicals in the denominator?

Yes, it is possible to have a limit with two radicals in the denominator. In this case, you can use algebraic manipulation to rationalize the denominator and simplify the expression before taking the limit.

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