Question Details

Let f : R { 1 2 } R and g : R { 5 2 } R be defined as f ( x ) = 2 x + 3 2 x + 1 and g ( x ) = | x | + 1 2 x + 5 then the domain of the function f(g(x)) is

Options

A

R

B

R { 5 2 }

C

R { 1 2 , 5 2 }

D

R { 1 2 }

Correct Answer :

R { 5 2 }

Solution :

The correct answer is:
R { 5 2 }

Step-by-Step Explanation:

To find the domain of the composite function f ( g ( x ) ) , we must satisfy two conditions:
1. x must lie in the domain of the inner function g ( x ) .
2. The value of g ( x ) must lie in the domain of the outer function f ( x ) .

Step 1: Determine the domain of g ( x )
The function g ( x ) is defined as:
g ( x ) = | x | + 1 2 x + 5
For g ( x ) to be real-valued and defined, its denominator must not be zero:
2 x + 5 0 x 5 2
Thus, the domain of g ( x ) is:
D g = R { 5 2 }

Step 2: Ensure g ( x ) lies in the domain of f ( x )
The domain of f ( x ) is R { 1 2 } . Therefore, we must exclude any values of x for which:
g ( x ) = 1 2
Let us solve the equation:
| x | + 1 2 x + 5 = 1 2
Cross-multiplying gives:
2 ( | x | + 1 ) = ( 2 x + 5 )
2 | x | + 2 = 2 x 5
Rearranging the terms:
2 | x | + 2 x = 7
2 ( | x | + x ) = 7

For any real number x , the absolute value | x | is always greater than or equal to x (i.e., | x | x ).
This implies that:
| x | + x 0
Consequently:
2 ( | x | + x ) 0
Since the left-hand side of our equation is always non-negative ( 0 ) and the right-hand side is strictly negative ( 7 ), there are no real values of x that satisfy this equation.
Therefore, g ( x ) 0 for all x > 5 2 , and g ( x ) < 0 for x < 5 2 , but g ( x ) never equals 1 2 for any real number x .

Conclusion:
Since no points from the domain of g ( x ) need to be excluded to satisfy the second condition, the domain of f ( g ( x ) ) is simply the domain of g ( x ) :
Domain = R { 5 2 }

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