Essentials of Calculus: find the limit problem

  • Thread starter Nawz
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  • #1
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Homework Statement



SYNTHESIS

Find limit as x approaches 3 of : (x^3-27)/x-3

Homework Equations





The Attempt at a Solution



I typed it into my calculator and got x^3 - (27)/(x-3) where x^3 wasn't divided by x-3. I really do not know where to start. I know the answer is 27 from the back of the book.

I was thinking it was something like x^3- (3)^3/ (x-3) and then you can cancel out one of the x-3 but I still cannot get 27 and if you plugin 3 into the original equation you get 0.
 
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Answers and Replies

  • #2
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Try factoring x3-27
 
  • #3
HallsofIvy
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You need to know that [itex](a- b)(a^2- ab+ b^2)= a^3- b^3[/itex].
 
  • #4
Or, if you ever forget that a³ - b³ = (whatever it actually equals :tongue2: )
It looks like something that can be factored.

Set x³ - 27 equal to zero.

x³ - 27 = 0

Find an x value that makes x³ - 27 = 0 hold, i.e. x = 3

(3)³ - 27 = 0

Then, because x = 3 is an answer, x - 3 = 0 is a factor so divide x - 3 into x³ - 27

______x²_+_..._______​
x - 3 |x³ + 0x² + 0x - 27
x³ - 3x²​
....​

Keep going & you'll factor it & get your answer :wink:
 
  • #5
hunt_mat
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Or failing that L'Hopital's rule...

Mat
 
  • #6
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Or use a substitution with u = x - 3, no factoring cubes or l'Hôpital's rule. :wink:
 

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