View Full Version : Evaluate the following limits
 
 Kan418
Jul 17, 2009, 03:32 AM
lim  x2(squared) - 6x + 8 / 2x2(squared) - 32
x-4  (the - is an arrow)
 iLLyriC
Jul 17, 2009, 05:52 AM
the easiest way is always L'ohspitals Rule:
Take the derivative of the numerator and denominator and  substitute x, in this case:
 
lim (x^2 - 6x +8)/2x^2 - 32  = lim (2x -6)/4x = 2/16 = 1/2
x->4
 ebaines
Jul 17, 2009, 08:47 AM
the easiest way is always L'ohspitals Rule:
Take the derivative of the numerator and denominator and  substitute x, in this case:
 
lim (x^2 - 6x +8)/2x^2 - 32  = lim (2x -6)/4x = 2/16 = 1/2
x->4
 
Right, except 2/16 = 1/8, not 1/2.
 iLLyriC
Jul 17, 2009, 02:01 PM
Right, except 2/16 = 1/8, not 1/2.
 LoL, thanks for correcting
 galactus
Jul 17, 2009, 02:14 PM
\lim_{x\to 4}\frac{x^{2}-6x+8}{2x^{2}-32}
 
If I may, L'Hopital is not needed in this case.  But it works OK.  
 
We are only dealing with a rational function. 
 
Notice we can factor:
 
\lim_{x\to 4} \;\ \frac{(x-4)(x-2)}{2(x-4)(x+4)}=\lim_{x\to 4} \;\ \frac{(x-2)}{2(x+4)}=\frac{1}{8}
 Kan418
Jul 20, 2009, 03:08 AM
Thank you Galactus.