Without using a calculator, solve the inequality xx−2<3(5x+2)(2−x)\frac{x}{x-2} < \frac{3}{(5x+2)(2-x)}. Hence, solve the following inequalities.

  1. (a)

    exex−2<3(5ex+2)(2−ex)\frac{\text{e}^x}{\text{e}^x-2} < \frac{3}{(5\text{e}^x+2)(2-\text{e}^x)}

  2. (b)

    xx−4>12(5x+4)(4−x)\frac{x}{x-4} > \frac{12}{(5x+4)(4-x)}

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Worked solution

3(a)

1. Solve the base inequality from the main stem, as its result is required for the 'Hence' part.

Since the TARGET depends on the result of the main stem, we must first solve xx−2<3(5x+2)(2−x)\frac{x}{x-2} < \frac{3}{(5x+2)(2-x)}.
xx−2−3(5x+2)(2−x)<0\frac{x}{x-2} - \frac{3}{(5x+2)(2-x)} < 0
xx−2+3(5x+2)(x−2)<0\frac{x}{x-2} + \frac{3}{(5x+2)(x-2)} < 0
x(5x+2)+3(x−2)(5x+2)<0\frac{x(5x+2) + 3}{(x-2)(5x+2)} < 0
5x2+2x+3(x−2)(5x+2)<0\frac{5x^2 + 2x + 3}{(x-2)(5x+2)} < 0
For the quadratic 5x2+2x+35x^2 + 2x + 3, the discriminant is 22−4(5)(3)=−56<02^2 - 4(5)(3) = -56 < 0. Since the coefficient of x2x^2 is 5>05 > 0, the numerator 5x2+2x+3>05x^2 + 2x + 3 > 0 for all real xx.
Therefore, the denominator must be negative for the fraction to be less than zero:
(x−2)(5x+2)<0(x-2)(5x+2) < 0
−25<x<2-\frac{2}{5} < x < 2

2. Substitute $x = \text{e}^x$ into the established solution.

By substituting xx with ex\text{e}^x into the solution of the base inequality, we get:
−25<ex<2-\frac{2}{5} < \text{e}^x < 2

3. Solve the resulting exponential inequality.

Since ex>0\text{e}^x > 0 for all real xx, the left inequality −25<ex-\frac{2}{5} < \text{e}^x is strictly true and imposes no restrictions.
We therefore only need to satisfy the right inequality:
ex<2\text{e}^x < 2
Taking the natural logarithm on both sides:
x<ln⁡2x < \ln 2

Answer

x<ln⁡2x < \ln 2

3(b)

1. Solve the original inequality (required prerequisite for the 'Hence' method).

Let the variable in the original inequality be uu. We are given:
uu−2−3(5u+2)(2−u)<0\frac{u}{u-2} - \frac{3}{(5u+2)(2-u)} < 0
u(5u+2)+3(u−2)(5u+2)<0\frac{u(5u+2)+3}{(u-2)(5u+2)} < 0
5u2+2u+3(u−2)(5u+2)<0\frac{5u^2+2u+3}{(u-2)(5u+2)} < 0
Since the discriminant of the numerator is 22−4(5)(3)=−56<02^2 - 4(5)(3) = -56 < 0 and the coefficient of u2u^2 is positive, 5u2+2u+3>05u^2+2u+3 > 0 for all real uu.
Thus, the inequality simplifies to the denominator being negative:
(u−2)(5u+2)<0(u-2)(5u+2) < 0
Which yields the solution:
−25<u<2-\frac{2}{5} < u < 2

2. Relate the target inequality to the original inequality by substitution.

Let u=x2u = \frac{x}{2}. Substituting this into the original expression gives:
LHS=x/2x/2−2=xx−4\text{LHS} = \frac{x/2}{x/2-2} = \frac{x}{x-4}
RHS=3(5(x/2)+2)(2−x/2)=12(5x+4)(4−x)\text{RHS} = \frac{3}{(5(x/2)+2)(2-x/2)} = \frac{12}{(5x+4)(4-x)}
Notice that the target inequality xx−4>12(5x+4)(4−x)\frac{x}{x-4} > \frac{12}{(5x+4)(4-x)} is identical to the original inequality evaluated at u=x2u = \frac{x}{2}, but with the inequality sign reversed (from << to >>).

3. Determine the solution set by applying the substitution and the reversed condition.

Since the original expression was <0< 0 for −25<u<2-\frac{2}{5} < u < 2, the reversed condition (>0> 0) holds for the outer regions:
u<−25oru>2u < -\frac{2}{5} \quad \text{or} \quad u > 2
Substitute u=x2u = \frac{x}{2} back into the inequalities:
x2<−25orx2>2\frac{x}{2} < -\frac{2}{5} \quad \text{or} \quad \frac{x}{2} > 2
x<−45orx>4x < -\frac{4}{5} \quad \text{or} \quad x > 4

Answer

x<−45 or x>4x < -\frac{4}{5} \text{ or } x > 4

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