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Evaluate this limit, if it exists

  • Thread starter ppkjref
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  • #1
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Homework Statement



Evaluate the limit, if it exists. the limit of ((2/(3+h))^2-(4/9))/h as h approaches 0.


The Attempt at a Solution



So far I got,

[(2/(3+h))^2-(4/9)]/h
= [(2/(3+h))(2/(3+h))-(4/9)]/h

It seems nothing can cancel out when I find a common denominator.
 

Answers and Replies

  • #2
Dick
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Keep going. You haven't done anything yet. Find a common denominator in the numerator and simplify it. Do some algebra.
 
  • #3
supratim1
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use L'Hospital rule
 
  • #4
supratim1
Gold Member
279
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note that its 0/0 form
 
  • #5
Dick
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use L'Hospital rule
That would work. But this looks like a difference quotient from a derivative definition. Probably wouldn't be appropriate to use l'Hopital.
 
  • #6
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i noted that it is 0/0 form so therefore I am likely to get a real number.
[(2/(3+h))^2-(4/9)]/h
= [(2/(3+h))(2/(3+h))-(4/9)]/h
=[(4/(9+6h+h^2))-(4/9)]/h
=[(36-(36+24h+4h^2))/(9(9+6h+h^2))]/h
=[(-4h(h+6))/((3h+9)(3h+9))]/h
=[(-4h^2(h+6))/((3h+9)(3h+9))]

So the denominator is 81 and the numerator is 0?
 
  • #7
supratim1
Gold Member
279
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That would work. But this looks like a difference quotient from a derivative definition. Probably wouldn't be appropriate to use l'Hopital.
yes agreed. if that's the case, then it can be solved by doing some algebra, would just be a little more lengthy. only if it is required not to be done by L'Hospital.
 
  • #8
Dick
Science Advisor
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i noted that it is 0/0 form so therefore I am likely to get a real number.
[(2/(3+h))^2-(4/9)]/h
= [(2/(3+h))(2/(3+h))-(4/9)]/h
=[(4/(9+6h+h^2))-(4/9)]/h
=[(36-(36+24h+4h^2))/(9(9+6h+h^2))]/h
=[(-4h(h+6))/((3h+9)(3h+9))]/h
=[(-4h^2(h+6))/((3h+9)(3h+9))]

So the denominator is 81 and the numerator is 0?
That's making my eyes hurt. Sorry. I think you were almost there in the fourth line. The 36's cancel. So you've got ((-24h+stuff)/(81+stuff))/h. Then a bad thing happened. How did the h pop up from the denominator into the numerator? It should have cancelled.
 
  • #9
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wow that's embarrassing. i'm an idiot hah
 

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