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Limit 2*x-sqrt(4*x^2+x+1)

  • Thread starter Suy
  • Start date
  • #1
Suy
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Homework Statement


lim x->-inf 2*x-sqrt(4*x^2+x+1)


Homework Equations





The Attempt at a Solution


For positive inf, i got -inf(not sure)
but negative inf, i got -1/4
http://www.wolframalpha.com/input/?i=lim+2*x-sqrt%284*x^2%2Bx%2B1%29
but wolfram have switch the answer, which is right, so i don't know which part i am wrong
lim X->-inf
2*x-sqrt(4*x^2+x+1)
2*x-sqrt(4*x^2+x+1)*((2*x+sqrt(4*x^2+x+1))/(2*x+sqrt(4*x^2+x+1))
(4x^2+4x^2-x-1)/(2*x-sqrt(4*x^2+x+1))
-x-1/(2*x-sqrt(4*x^2+x+1))
-x/(2x-sqrt(4x^2) X<0 sqrt(4x^2) =|2x|=-2x
-x/(2x-(-2x))=-1/4

lim x->+inf
-x/(2x-sqrt(4x^2) X>0 sqrt(4x^2) =|2x|=2x
-x/0= -inf?
Thanks
 

Answers and Replies

  • #2
Mentallic
Homework Helper
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2*x-sqrt(4*x^2+x+1)
2*x-sqrt(4*x^2+x+1)*((2*x+sqrt(4*x^2+x+1))/(2*x+sqrt(4*x^2+x+1))
Yes, good.

(4x^2+4x^2-x-1)/(2*x-sqrt(4*x^2+x+1))
No, you should have [tex]\frac{4x^2-(4x^2+x+1)}{2x+\sqrt{4x^2+x+1}}[/tex]
You got the numerator wrong - specifically it should be -4x2 not +4x2. And in the denominator, you changed it to a minus when you showed a plus in the previous line.
 

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