An algebra problem by Gazar Khalid

Algebra Level 3

If f ( x ) = x + 6 f(x) = \sqrt{x+6} and g ( x ) = x + 9 x 1 g(x) = \dfrac{x+9}{x-1} , find the domain of f ( x ) g ( x ) \dfrac{f(x)}{g(x)} .

( 9 , 1 ) ( 1 , ) (-9,1) \cap (1,\infty) [ 6 , ) [-6,\infty) ( 6 , 1 ) ( 1 , ) (-6, 1) \cup (1, \infty) [ 6 , 1 ) ( 1 , ) [-6,1) \cup (1,\infty)

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2 solutions

Mahdi Raza
Jun 30, 2020

We know:

Domain of ( f g ) ( x ) = Domain of f ( x ) Domain of g ( x ) \text{Domain of } \left(\dfrac{f}{g}\right)(x) = \text{Domain of } f(x) \cap \text{Domain of } g(x)

The domain of f ( x ) f(x) :

Domain of f ( x ) x + 6 0 x 6 x [ 6 , ) \begin{aligned} \text{Domain of } f(x) \implies \sqrt{x + 6} &\geq 0 \\ x &\geq -6 \\ x &\in [-6, \infty) \end{aligned}

The domain of g ( x ) g(x) :

Domain of g ( x ) x + 9 x 1 ( x 1 ) 0 x R { 1 } Not defined for x-1 \begin{aligned} \text{Domain of } g(x) \implies \dfrac{x+9}{x-1} &\blue{(x-1) \ne 0} \\ x \in \R - \{1\} &\blue{\text{Not defined for x-1}} \end{aligned}

Now intersection of values is:

x [ 6 , ) x R { 1 } x [ 6 , ) { 1 } x \in [-6, \infty) \cap x \in \R - \{1\} \quad \implies \quad x \in [-6, \infty) - \{1\} x [ 6 , 1 ) ( 1 , ) x \in \boxed{[-6, 1) \cup (1, \infty)}

The domain of f ( x ) f(x) is x [ 6 , ) x \in [-6, \infty) . The domain of g ( x ) g(x) is x ( , 1 ) ( 1 , ) x \in (-\infty, 1) \cup (1, \infty) . Therefore, the domain of f ( x ) g ( x ) \dfrac {f(x)}{g(x)} is x [ 6 , 1 ) ( 1 , ) x \in \boxed{[-6, 1) \cup (1, \infty)} .

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