Without using a graphic calculator, solve the inequality x+22x−1<2x+1\frac{x+2}{2x-1} < 2x+1.

  1. Hence find the exact solution of the inequality 2x2+12−x2<2+x2x2\frac{2x^2+1}{2-x^2} < \frac{2+x^2}{x^2}.

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Worked solution

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1. Solve the inequality from the first part.

x+22x−1<2x+1\frac{x+2}{2x-1} < 2x+1
x+22x−1−(2x+1)(2x−1)2x−1<0\frac{x+2}{2x-1} - \frac{(2x+1)(2x-1)}{2x-1} < 0
x+2−(4x2−1)2x−1<0\frac{x+2 - (4x^2 - 1)}{2x-1} < 0
−4x2+x+32x−1<0\frac{-4x^2 + x + 3}{2x-1} < 0
4x2−x−32x−1>0\frac{4x^2 - x - 3}{2x-1} > 0
(4x+3)(x−1)2x−1>0\frac{(4x+3)(x-1)}{2x-1} > 0
The critical values are x=−34x = -\frac{3}{4}, x=12x = \frac{1}{2}, and x=1x = 1.
By performing a sign test, the solution is:
−34<x<12orx>1-\frac{3}{4} < x < \frac{1}{2} \quad \text{or} \quad x > 1

2. Relate the target inequality to the first part using a substitution.

Let y=1x2y = \frac{1}{x^2}. We rewrite the left-hand side of the target inequality:
2x2+12−x2=2(1y)+12−1y=2+yy2y−1y=y+22y−1\frac{2x^2+1}{2-x^2} = \frac{2\left(\frac{1}{y}\right)+1}{2-\frac{1}{y}} = \frac{\frac{2+y}{y}}{\frac{2y-1}{y}} = \frac{y+2}{2y-1}
The right-hand side becomes:
2+x2x2=2x2+1=2y+1\frac{2+x^2}{x^2} = \frac{2}{x^2} + 1 = 2y + 1
Thus, the inequality 2x2+12−x2<2+x2x2\frac{2x^2+1}{2-x^2} < \frac{2+x^2}{x^2} is exactly equivalent to:
y+22y−1<2y+1\frac{y+2}{2y-1} < 2y+1

3. Use the solution from the first part to set up inequalities for $x$.

Since this matches the inequality solved in the first part, the solution for yy is:
−34<y<12ory>1-\frac{3}{4} < y < \frac{1}{2} \quad \text{or} \quad y > 1
Substituting y=1x2y = \frac{1}{x^2} back into the inequalities:
−34<1x2<12or1x2>1-\frac{3}{4} < \frac{1}{x^2} < \frac{1}{2} \quad \text{or} \quad \frac{1}{x^2} > 1

4. Solve the resulting inequalities for $x$, noting that $x^2 > 0$.

Since x2>0x^2 > 0 for all real x≠0x \neq 0, the term 1x2\frac{1}{x^2} is always strictly positive.
For the first inequality:
0<1x2<12  ⟹  x2>20 < \frac{1}{x^2} < \frac{1}{2} \implies x^2 > 2
x<−2orx>2x < -\sqrt{2} \quad \text{or} \quad x > \sqrt{2}
For the second inequality:
1x2>1  ⟹  x2<1  ⟹  −1<x<1\frac{1}{x^2} > 1 \implies x^2 < 1 \implies -1 < x < 1
Since x≠0x \neq 0 (otherwise 1x2\frac{1}{x^2} is undefined), this yields:
−1<x<0or0<x<1-1 < x < 0 \quad \text{or} \quad 0 < x < 1
Combining all parts, the exact solution is:
x<−2,−1<x<0,0<x<1orx>2x < -\sqrt{2}, \quad -1 < x < 0, \quad 0 < x < 1 \quad \text{or} \quad x > \sqrt{2}

Answer

x<−2x < -\sqrt{2}, −1<x<0-1 < x < 0, 0<x<10 < x < 1 or x>2x > \sqrt{2}

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