Saturday, July 27, 2013

Find the limit (x^2+x-6)/(x+3) when x approach -3.

For evaluating the limit, we'll choose the dividing out technique.


We'll apply the direct substitution, by substituting the unknown x, by the value -3 and we'll see that it fails, because both, numerator and denominator, are cancelling for x=-3. That means x=-3 is a root for both, that means that (x+3) is a common factor for both.


We'll write the numerator using the formula:


x^2+x-6=(x-x1)(x-x2), where x1, x2 are the roots and x1=-3


x^2+x-6=(x+3)(x-x2)


We also know that x1+x2 = -1, -3+x2=-1


and x1*x2=-6, (-3)*x2=-6


x2=2


Now, we'll evaluate the limit:


lim (x^2+x-6)/(x+3) = lim (x+3)(x-2)/(x+3)


Now, we can divide out like factor:


lim (x^2+x-6)/(x+3) = lim (x-2)


We can apply the replacement theorem and we'll get:


lim (x-2) = -3-2 = -5


So, lim (x^2+x-6)/(x+3) = -5.

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