Evaluate Lim → 0, radicals in the numerator and denominator

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In summary, the limit of √(x+1) - √(2x+1) / √(3x+4) - √(2x+4) as x approaches 0 is equal to -1/4. This was evaluated using L'Hôpital's rule.
  • #1
physphys
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Homework Statement



Evaluate lim x →0 √(x+1) - √(2x+1)
-----------------
√(3x+4) - √(2x+4)

Homework Equations





The Attempt at a Solution



First I rationalized the numerator by multiplying everything by √(x+1) + √(2x+1) / √(x+1) + √(2x+1)

so now i have
-x
----
( √(3x+4) - √(2x+4) )(√(x+1) + √(2x+1))


and I'm stuck from here, the answer is -2 so I'm not sure if I'm even right so far.
Thanks in advance for the help!
 
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  • #2
Try multiplying the numerator and denominator by ##\sqrt{3x+4}+\sqrt{2x+4}##?
 
  • #3
Try L'Hôpital's rule.
 
  • #4
Well we never learned l'hopital's rule yet but after googling it and attempting it i got
(1/2) - 1
----------
(3/2) - (1/2)
which gave me -1/4, not the answer.

Now as for jbunnii's plan. I got up to
- x ( (√3x+1) + (√2x+4) )
------------------------
(x-3) ((√x+1) + (√2x+1))

I'm unsure of what to do next.

Edit: Can the X's cancel out here? so I end up with
- ( (√3x+1) + (√2x+4) )
------------------------
-3 ((√x+1) + (√2x+1))

Edit 2 : No that's giving me - 1/2, the inverse of what I need
 
  • #5
physphys said:
Well we never learned l'hopital's rule yet but after googling it and attempting it i got
(1/2) - 1
----------
(3/2) - (1/2)
which gave me -1/4, not the answer.

How can you get -1/4? Anyway, the (3/2) in the denominator is wrong.
 
  • #6
physphys said:
Well we never learned l'hopital's rule yet but after googling it and attempting it i got
(1/2) - 1
----------
(3/2) - (1/2)
which gave me -1/4, not the answer.

Now as for jbunnii's plan. I got up to
- x ( (√3x+1) + (√2x+4) )
------------------------
(x-3) ((√x+1) + (√2x+1))
How did you end up with ##x-3## in the denominator? I calculate
$$(\sqrt{3x+4} - \sqrt{2x+4})(\sqrt{3x+4} + \sqrt{2x+4}) = (3x+4) - (2x+4) = x$$
Now cancel the ##x## in the numerator and denominator and see what is left.
 
  • #7
Also, you somehow changed your ##\sqrt{3x+4}## into ##\sqrt{3x+1}##.
 
  • #8
jbunniii said:
How did you end up with ##x-3## in the denominator? I calculate
$$(\sqrt{3x+4} - \sqrt{2x+4})(\sqrt{3x+4} + \sqrt{2x+4}) = (3x+4) - (2x+4) = x$$
Now cancel the ##x## in the numerator and denominator and see what is left.

yeah you're right, I got the answer! I just copied the question down wrong. Thanks for the help everyone!
 

Related to Evaluate Lim → 0, radicals in the numerator and denominator

1. What is the definition of a limit?

A limit is the value that a function approaches as the input variable approaches a certain value.

2. How do you evaluate a limit with radicals in the numerator and denominator?

To evaluate a limit with radicals in the numerator and denominator, you can use the rationalizing technique. This involves multiplying the numerator and denominator by the conjugate of the radical expression in order to eliminate the radicals.

3. Can a limit with radicals in the numerator and denominator be simplified?

Yes, a limit with radicals in the numerator and denominator can be simplified by using algebraic techniques such as factoring and canceling out common factors.

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

A one-sided limit only considers the values approaching from one direction, either from the left or the right. A two-sided limit takes into account both directions and the function must approach the same value from both sides in order for the limit to exist.

5. Are there any special rules for evaluating limits involving radicals?

Yes, there are some special rules for evaluating limits with radicals. For example, the limit of the square root of x as x approaches 0 is equal to 0, while the limit of the cube root of x as x approaches 0 is equal to 0. These rules can be helpful when evaluating more complex limits involving radicals.

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