^{2 }- 4x - 12

Find the domain of the rational

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Kevin F. | Computer Programming and Mathematics TutorComputer Programming and Mathematics Tut...

The domain means the values of x that are allowed. In the case of rational expressions, that is all values that don't result in division by zero.

x - 7

-------------

x^{2 }- 4x - 12

x - 7

= ----------------

(x - 6) (x + 2)

The domain is all values of x except for -2 and 6.

I assume you mean (x-7) / (x^2 - 4x - 12), right?

If so the first step is to factor the quadratic expression in the denominator. It factors into (x-6)(x+2).

Since there are no (binomial) factors in common between the numerator and the denominator, this expression can not be further simplified.

The domain for a polynomial expression, like the this one, is the set of all real numbers except for those values that would make the denominator equal to zero. In this specific case, if x = 6 or x = -2, the denominator would equal zero.

So, the domain of this function is (- infinity, -2) U (-2, 6) U (6, positive infinity).

In our case we have to exclude the points where trinomial equal zero.

x - 7

f(x) = ---------------

x

x

x ≠ 6

x ≠ - 2

Thus, domain is (- ∞, - 2) U (-2, 6) U (6, + ∞) , in other words: all real number except "- 2" and 6 .

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