Keep the denominator sign in view

Solve (2x−18)/(x+3)>x−6(2x-18)/(x+3)>x-6 algebraically. marks Hence solve (ln⁡(x2)−18)/(ln⁡x+3)>ln⁡x−6(\ln(x^2)-18)/(\ln x+3)>\ln x-6. marks

[2009/JJC/Prelim/I/6b]

Total marks: 6

Solution

(a)

Rational inequalities

Solve the inequality 2x−18x+3>x−6\dfrac{2x-18}{x+3}>x-6 algebraically.

Explore this idea

Hint

  1. Do not cross-multiply: x+3x+3 can be positive or negative.

  2. Move everything to the left and combine into one fraction.

  3. Factorise and use a sign chart; remember x≠−3x\ne-3.

Answer · (a)

x<−3x<-3 or 0<x<50<x<5.

Explore the idea

Where is x(x − 5)/(x + 3) negative?

Drag the test value. Each factor has a sign; the product's sign decides the inequality.

x(x−5)x+3<0\frac{x(x-5)}{x+3}<0

Choose an interval to test it. Shading marks the solution intervals.

Intervalxx − 5x + 3product
negativenegativenegativenegative
negativenegativepositivepositive
positivenegativepositivenegative
positivepositivepositivepositive
Test value of xx = 2.00
Try an example

(x)+(x − 5)−(x + 3)+⇒ product negative

Is it < 0?
Yes
x is in the solution set

The shaded strips are where the expression satisfies the inequality; they match the green-tinted columns of the chart.

  • Expression, and the x-values where it is negative
  • Your test value
Key idea

The product changes sign at each simple root. With three simple roots, the sign alternates, and the answer is the negative intervals.

Ideas to try
  1. Test x=−4x=-4, −1-1, 22 and 66. Which are solutions?
  2. Move the test value across x=−3x=-3: what happens to the denominator's sign?
(b)

Substitution into a known result

Hence solve ln⁡(x2)−18ln⁡x+3>ln⁡x−6\dfrac{\ln(x^2)-18}{\ln x+3}>\ln x-6.

Hint

  1. Use ln⁡(x2)=2ln⁡x\ln(x^2)=2\ln x.

  2. Compare with part (a): ln⁡x\ln x plays the role of xx.

  3. Translate back to xx using exponentials. Keep x>0x>0.

Answer · (b)

0<x<e−30<x<e^{-3} or 1<x<e51<x<e^5.