Find the set of values of xx such that −1<2x+3x−1<1-1<\dfrac{2x+3}{x-1}<1. Deduce the range of values of xx such that −1<2x+1x−2<1-1<\dfrac{2x+1}{x-2}<1.

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Worked solution

Q1

1. Transform the rational inequality into a quadratic inequality by squaring.

Since the inequality is of the form −1<expression<1-1 < \text{expression} < 1, it is equivalent to ∣expression∣<1|\text{expression}| < 1. We can square both sides:
−1<2x+3x−1<1-1 < \frac{2x+3}{x-1} < 1
(2x+3x−1)2<1\left( \frac{2x+3}{x-1} \right)^2 < 1
(2x+3)2<(x−1)2(2x+3)^2 < (x-1)^2

2. Solve the quadratic inequality to find the solution set for the first part.

4x2+12x+9<x2−2x+14x^2 + 12x + 9 < x^2 - 2x + 1
3x2+14x+8<03x^2 + 14x + 8 < 0
(3x+2)(x+4)<0(3x+2)(x+4) < 0

Therefore, the set of values is:
−4<x<−23-4 < x < -\frac{2}{3}

3. Identify the relationship between the expressions in the first and second inequalities.

Let f(x)=2x+3x−1\mathrm{f}(x) = \dfrac{2x+3}{x-1}.
We want to solve −1<f(X)<1-1 < \mathrm{f}(X) < 1 where f(X)=2x+1x−2\mathrm{f}(X) = \dfrac{2x+1}{x-2}.
Notice that substituting X=x−1X = x-1 into f(X)\mathrm{f}(X) gives:
f(x−1)=2(x−1)+3(x−1)−1=2x−2+3x−2=2x+1x−2\mathrm{f}(x-1) = \frac{2(x-1)+3}{(x-1)-1} = \frac{2x-2+3}{x-2} = \frac{2x+1}{x-2}

4. Use the substitution to deduce the range of values for the second inequality.

From the first part, the solution to −1<f(X)<1-1 < \mathrm{f}(X) < 1 is −4<X<−23-4 < X < -\frac{2}{3}.
Substituting X=x−1X = x-1 into this range:
−4<x−1<−23-4 < x-1 < -\frac{2}{3}
−3<x<13-3 < x < \frac{1}{3}

Answer

−4<x<−23-4 < x < -\frac{2}{3}

−3<x<13-3 < x < \frac{1}{3}

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