The function gg is defined by g:x↦ln⁡(x−3)g : x \mapsto \ln(x - \sqrt{3}), for x∈Rx \in \mathbb{R}, x>3x > \sqrt{3}.

  1. (a)

    Explain how you know g−1g^{-1} exists.

  2. (b)

    Find g−1(x)g^{-1}(x) and write down its domain.

  3. (c)

    Solve g(x)=0g(x) = 0 exactly.

  4. (d)

    Hence, without using a calculator, solve the inequality g(x)(x−3)(x+1)≥0\displaystyle \frac{g(x)}{(x-3)(x+1)} \ge 0.

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Worked solution

8(a)

1. Show that the function evaluates to the same output only if the inputs are identical.

Let g(a)=g(b)g(a) = g(b) for some a,b>3a, b > \sqrt{3}.
ln⁡(a−3)=ln⁡(b−3)\ln(a - \sqrt{3}) = \ln(b - \sqrt{3})
a−3=b−3a - \sqrt{3} = b - \sqrt{3}
a=ba = b

2. Conclude that the function is one-to-one, satisfying the condition for an inverse to exist.

Since g(a)=g(b)  ⟹  a=bg(a) = g(b) \implies a = b, gg is a one-to-one function.
Therefore, g−1g^{-1} exists.

Answer

gg is a one-to-one function, hence g−1g^{-1} exists.

8(b)

1. Set $y = g(x)$ and make $x$ the subject.

Let y=ln⁡(x−3)y = \ln(x - \sqrt{3}).
ey=x−3e^y = x - \sqrt{3}
x=ey+3x = e^y + \sqrt{3}

2. State the inverse function $g^{-1}(x)$.

g−1(x)=ex+3g^{-1}(x) = e^x + \sqrt{3}

3. Determine the domain of $g^{-1}$ by considering the range of $g$.

The domain of g−1g^{-1} is exactly the range of gg.
For x>3x > \sqrt{3}, the expression x−3x - \sqrt{3} takes all values in (0,∞)(0, \infty).
Therefore, ln⁡(x−3)\ln(x - \sqrt{3}) takes all real values.
Domain of g−1 is x∈R\text{Domain of } g^{-1} \text{ is } x \in \mathbb{R}

Answer

g−1(x)=ex+3g^{-1}(x) = e^x + \sqrt{3}, Domain: x∈Rx \in \mathbb{R}

8(c)

1. Set up the equation to solve for $x$.

g(x)=0g(x) = 0
ln⁡(x−3)=0\ln(x - \sqrt{3}) = 0

2. Convert the logarithmic equation to an exponential form to find the exact value of $x$.

x−3=e0x - \sqrt{3} = e^0
x−3=1x - \sqrt{3} = 1
x=1+3x = 1 + \sqrt{3}

Answer

x=1+3x = 1 + \sqrt{3}

8(d)

1. Establish the domain of the inequality and the root from the numerator.

From the definition of g(x)g(x), the domain is x>3x > \sqrt{3}.
To find the critical point from the numerator, we solve g(x)=0g(x) = 0 (as calculated in part c):
ln⁡(x−3)=0  ⟹  x−3=1  ⟹  x=1+3\ln(x - \sqrt{3}) = 0 \implies x - \sqrt{3} = 1 \implies x = 1 + \sqrt{3}

2. Simplify the inequality by identifying any strictly positive factors within the domain.

For the domain x>3x > \sqrt{3}, the factor (x+1)(x+1) in the denominator is strictly positive, since x+1>3+1>0x + 1 > \sqrt{3} + 1 > 0.
Thus, we can multiply both sides of the inequality by x+1x+1 without reversing the inequality sign, simplifying it to:
g(x)x−3≥0\frac{g(x)}{x-3} \ge 0

3. Identify the remaining critical value and determine their relative order without a calculator.

We identify the remaining critical value from the denominator as x=3x = 3.
To construct a sign test without a calculator, we determine the relative order of the domain boundary 3\sqrt{3} and the critical values 1+31+\sqrt{3} and 33.
Since 1<3<4=21 < \sqrt{3} < \sqrt{4} = 2, it logically follows that:
1+3<1+2=31 + \sqrt{3} < 1 + 2 = 3
So the critical points in ascending order are 3\sqrt{3}, 1+31+\sqrt{3}, and 33.

4. Perform a sign test on the valid intervals and deduce the final solution.

We perform a sign test on the intervals bounded by the domain and critical values:

  • For 3<x<1+3\sqrt{3} < x < 1+\sqrt{3}: g(x)<0g(x) < 0 and x−3<0  ⟹  g(x)x−3>0x-3 < 0 \implies \frac{g(x)}{x-3} > 0
  • For 1+3<x<31+\sqrt{3} < x < 3: g(x)>0g(x) > 0 and x−3<0  ⟹  g(x)x−3<0x-3 < 0 \implies \frac{g(x)}{x-3} < 0
  • For x>3x > 3: g(x)>0g(x) > 0 and x−3>0  ⟹  g(x)x−3>0x-3 > 0 \implies \frac{g(x)}{x-3} > 0

The inequality includes equality to 00, which occurs at the numerator's zero, x=1+3x = 1+\sqrt{3}. At x=3x = 3, the denominator is zero so the expression is undefined.
Combining these observations gives the final valid intervals.

Answer

\sqrt{3} < x \le 1+\sqrt{3} \text{ or } x > 3

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