Functions
Composite Functions / Domain
GRB_1000_SCQ
Grade Class 11
Question:
If $f$ is a function with domain $[-3, 5]$ and $g(x) = |3x + 4|$, then the domain of $(fog)(x)$ is:
$\left(-3, \frac{1}{3}\right)$
$\left[-3, \frac{1}{3}\right]$
$\left[-3, \frac{1}{3}\right]$
$\left[-3, \frac{-1}{3}\right]$
Step-by-Step Solution
Key Concept: Domain of composite function $fog$ requires $g(x)$ to lie in the domain of $f$.
Step 1: Understand the composition requirement.
For the composite function $(fog)(x) = f(g(x))$ to be defined, the output of $g(x)$ must lie within the domain of $f$. Since the domain of $f$ is $[-3, 5]$, we need:
$$-3 \leq g(x) \leq 5$$
Step 2: Analyze the lower bound condition.
We need to check if $g(x) \geq -3$. Since $g(x) = |3x + 4|$ and absolute values are always non-negative:
$$g(x) = |3x + 4| \geq 0$$
Since $0 > -3$, the condition $g(x) \geq -3$ is automatically satisfied for all real $x$. Therefore, we only need to focus on the upper bound.
Step 3: Apply the upper bound condition.
We need $g(x) \leq 5$, which means:
$$|3x + 4| \leq 5$$
Step 4: Solve the absolute value inequality.
The inequality $|3x + 4| \leq 5$ is equivalent to:
$$-5 \leq 3x + 4 \leq 5$$
Step 5: Isolate the variable $x$.
Subtract 4 from all parts:
$$-5 - 4 \leq 3x \leq 5 - 4$$
$$-9 \leq 3x \leq 1$$
Divide all parts by 3:
$$-3 \leq x \leq \frac{1}{3}$$
Step 6: State the final answer.
The domain of $(fog)(x)$ is the set of all $x$ values satisfying the above inequality:
$$\boxed{\left[-3, \frac{1}{3}\right]}$$
This matches **Option 2** (and Option 3, as they are identical).
Correct Answer: 2