The function f is defined, for x>32x > \frac{3}{2}, by f(x)=e2x1−x\text{f}(x) = \frac{e^{2x}}{1 - x}. Explain clearly whether f is an increasing or a decreasing function.

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Worked solution

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1. Find the first derivative of $f(x)$ using the quotient rule.

f′(x)=(1−x)ddx(e2x)−e2xddx(1−x)(1−x)2\text{f}'(x) = \frac{(1 - x)\frac{d}{dx}(e^{2x}) - e^{2x}\frac{d}{dx}(1 - x)}{(1 - x)^2}
f′(x)=(1−x)(2e2x)−e2x(−1)(1−x)2\text{f}'(x) = \frac{(1 - x)(2e^{2x}) - e^{2x}(-1)}{(1 - x)^2}

2. Simplify the derivative.

f′(x)=2e2x−2xe2x+e2x(1−x)2\text{f}'(x) = \frac{2e^{2x} - 2xe^{2x} + e^{2x}}{(1 - x)^2}
f′(x)=e2x(3−2x)(1−x)2\text{f}'(x) = \frac{e^{2x}(3 - 2x)}{(1 - x)^2}

3. Analyse the sign of the derivative for the given domain.

For x>32x > \frac{3}{2}:

  • e2x>0e^{2x} > 0 for all real xx.
  • (1−x)2>0(1 - x)^2 > 0 since x≠1x \neq 1.
  • 3−2x<03 - 2x < 0 because 2x>32x > 3.
    Therefore, f′(x)=(+)(−)(+)<0\text{f}'(x) = \frac{(+)(-)}{(+)} < 0.

4. State the conclusion.

Since f′(x)<0\text{f}'(x) < 0 for all x>32x > \frac{3}{2}, f\text{f} is a decreasing function.

Answer

Since f′(x)<0f'(x) < 0 for x>32x > \frac{3}{2}, ff is a decreasing function.

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