The function is defined by
- (i)
Explain why the function does not exist.
- (ii)
Find, algebraically, the set of values of for which is decreasing.
- (iii)
In the rest of the question, the domain of is further restricted to .
The function is defined by
Find an expression for and hence, or otherwise, find .
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Worked solution
9(i)
1. Evaluate the function at two distinct points to find a counterexample for the one-to-one property.
Let ,
Let ,
2. Conclude that the function is not one-to-one, meaning its inverse does not exist.
Since for , the function is not a one-to-one function.
Therefore, the inverse function does not exist.
Since , the function is not one-to-one. Therefore, does not exist.
9(ii)
1. Find the first derivative of $\text{f}(x)$.
Rewrite using a negative index:
Differentiate with respect to using the chain rule:
2. Set up the inequality for a decreasing function.
For the function to be decreasing, its gradient must be negative, so .
3. Solve the rational inequality.
The denominator is a perfect square, meaning for all valid in the domain (where and ).
Therefore, the sign of the derivative depends entirely on the numerator. For the fraction to be negative, we must have:
4. Combine with the domain restrictions to state the final set of values.
The given domain restricts and .
The condition already excludes . However, is greater than and thus falls within this region, so it must still be explicitly excluded.
The set of values is therefore .
(Note: is also acceptable depending on the rigorous definition of decreasing vs strictly decreasing)
9(iii)
1. Find an expression for $\text{gf}(x)$ by substituting $\text{f}(x)$ into $\text{g}(x)$.
2. Set up an equation to evaluate the inverse function at $\frac{1}{4}$ and solve the resulting quadratic equation.
Let .
3. Determine the valid root by considering the domain restriction.
Since the domain of is restricted to , the domain of is also .
We reject .
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