函数 f\text{f} 的定义为
f:x↦1x2−x−6+2,x∈R, x≠−2, x≠3.\text{f} : x \mapsto \frac{1}{x^2 - x - 6} + 2, \quad x \in \mathbb{R}, \ x \ne -2, \ x \ne 3.

  1. (i)

    解释为什么函数 f−1\text{f}^{-1} 不存在。

  2. (ii)

    用代数方法求使 f\text{f} 单调递减的 xx 的取值集合。

  3. (iii)

    在问题的其余部分中,f\text{f} 的定义域进一步限制为 x≤12x \le \frac{1}{2}。

    函数 g\text{g} 的定义为
    g:x↦2−x,x∈R.\text{g} : x \mapsto 2 - x, \quad x \in \mathbb{R}.

    求 gf(x)\text{gf}(x) 的表达式,并由此或用其他方法求 (gf)−1(14)(\text{gf})^{-1}\left(\frac{1}{4}\right) 的值。

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完整解法

9(i)

1. Evaluate the function at two distinct points to find a counterexample for the one-to-one property.

Let x=0x = 0,
f(0)=102−0−6+2=−16+2=116\text{f}(0) = \frac{1}{0^2 - 0 - 6} + 2 = -\frac{1}{6} + 2 = \frac{11}{6}

Let x=1x = 1,
f(1)=112−1−6+2=−16+2=116\text{f}(1) = \frac{1}{1^2 - 1 - 6} + 2 = -\frac{1}{6} + 2 = \frac{11}{6}

2. Conclude that the function is not one-to-one, meaning its inverse does not exist.

Since f(0)=f(1)\text{f}(0) = \text{f}(1) for 0≠10 \ne 1, the function f\text{f} is not a one-to-one function.

Therefore, the inverse function f−1\text{f}^{-1} does not exist.

答案

Since f(0)=f(1)=116\text{f}(0) = \text{f}(1) = \frac{11}{6}, the function f\text{f} is not one-to-one. Therefore, f−1\text{f}^{-1} does not exist.

9(ii)

1. Find the first derivative of $\text{f}(x)$.

Rewrite f(x)\text{f}(x) using a negative index:
f(x)=(x2−x−6)−1+2\text{f}(x) = (x^2 - x - 6)^{-1} + 2

Differentiate with respect to xx using the chain rule:
f′(x)=−(x2−x−6)−2(2x−1)\text{f}'(x) = -(x^2 - x - 6)^{-2}(2x - 1)
f′(x)=1−2x(x2−x−6)2\text{f}'(x) = \frac{1 - 2x}{(x^2 - x - 6)^2}

2. Set up the inequality for a decreasing function.

For the function to be decreasing, its gradient must be negative, so f′(x)<0\text{f}'(x) < 0.
1−2x(x2−x−6)2<0\frac{1 - 2x}{(x^2 - x - 6)^2} < 0

3. Solve the rational inequality.

The denominator (x2−x−6)2(x^2 - x - 6)^2 is a perfect square, meaning (x2−x−6)2>0(x^2 - x - 6)^2 > 0 for all valid xx in the domain (where x≠−2x \ne -2 and x≠3x \ne 3).

Therefore, the sign of the derivative depends entirely on the numerator. For the fraction to be negative, we must have:
1−2x<01 - 2x < 0
2x>1  ⟹  x>122x > 1 \implies x > \frac{1}{2}

4. Combine with the domain restrictions to state the final set of values.

The given domain restricts x≠−2x \ne -2 and x≠3x \ne 3.
The condition x>12x > \frac{1}{2} already excludes −2-2. However, x=3x = 3 is greater than 12\frac{1}{2} and thus falls within this region, so it must still be explicitly excluded.

The set of values is therefore x>12,x≠3x > \frac{1}{2}, x \ne 3.
(Note: x≥12,x≠3x \ge \frac{1}{2}, x \ne 3 is also acceptable depending on the rigorous definition of decreasing vs strictly decreasing)

答案

x>12,x≠3x > \frac{1}{2}, x \ne 3

9(iii)

1. Find an expression for $\text{gf}(x)$ by substituting $\text{f}(x)$ into $\text{g}(x)$.

gf(x)=g(1x2−x−6+2)\text{gf}(x) = \text{g}\left( \frac{1}{x^2 - x - 6} + 2 \right)
gf(x)=2−(1x2−x−6+2)\text{gf}(x) = 2 - \left( \frac{1}{x^2 - x - 6} + 2 \right)
gf(x)=−1x2−x−6\text{gf}(x) = -\frac{1}{x^2 - x - 6}

2. Set up an equation to evaluate the inverse function at $\frac{1}{4}$ and solve the resulting quadratic equation.

Let x=(gf)−1(14)  ⟹  gf(x)=14x = (\text{gf})^{-1}\left(\frac{1}{4}\right) \implies \text{gf}(x) = \frac{1}{4}.
−1x2−x−6=14-\frac{1}{x^2 - x - 6} = \frac{1}{4}
x2−x−6=−4x^2 - x - 6 = -4
x2−x−2=0x^2 - x - 2 = 0
(x−2)(x+1)=0(x - 2)(x + 1) = 0
x=2orx=−1x = 2 \quad \text{or} \quad x = -1

3. Determine the valid root by considering the domain restriction.

Since the domain of f\text{f} is restricted to x≤12x \le \frac{1}{2}, the domain of gf\text{gf} is also x≤12x \le \frac{1}{2}.
We reject x=2x = 2.
∴(gf)−1(14)=−1\therefore (\text{gf})^{-1}\left(\frac{1}{4}\right) = -1

答案

gf(x)=−1x2−x−6\text{gf}(x) = -\frac{1}{x^2 - x - 6}
(gf)−1(14)=−1(\text{gf})^{-1}\left(\frac{1}{4}\right) = -1

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