Show algebraically that x2+2x+2x^2 + 2x + 2 is always positive for all real values of xx. Without using a calculator, solve the inequality x2+2x+26−x−x2≤0\frac{x^2 + 2x + 2}{6 - x - x^2} \le 0. Hence, solve the inequality x+2x+2x+x−6≥0\frac{x + 2\sqrt{x} + 2}{x + \sqrt{x} - 6} \ge 0.

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Worked solution

4

1. Show that the quadratic expression is always positive by completing the square.

x2+2x+2=(x+1)2−1+2=(x+1)2+1x^2 + 2x + 2 = (x+1)^2 - 1 + 2 = (x+1)^2 + 1
Since (x+1)2≥0(x+1)^2 \ge 0 for all real values of xx, it follows that (x+1)2+1≥1>0(x+1)^2 + 1 \ge 1 > 0.
Thus, x2+2x+2x^2 + 2x + 2 is always positive.

2. Solve the rational inequality by determining the sign of the denominator.

For the inequality x2+2x+26−x−x2≤0\frac{x^2 + 2x + 2}{6 - x - x^2} \le 0, since the numerator x2+2x+2>0x^2 + 2x + 2 > 0, the inequality is satisfied if and only if the denominator is strictly negative (it cannot be zero).
6−x−x2<06 - x - x^2 < 0
x2+x−6>0x^2 + x - 6 > 0
(x+3)(x−2)>0(x+3)(x-2) > 0
This gives the solution x<−3 or x>2x < -3 \text{ or } x > 2.

3. Rewrite the third inequality to match the form of the second inequality.

The given inequality is x+2x+2x+x−6≥0\frac{x + 2\sqrt{x} + 2}{x + \sqrt{x} - 6} \ge 0.
Multiply the numerator and denominator by −1-1 to rearrange the denominator:
(x)2+2x+2−(6−x−(x)2)≥0\frac{(\sqrt{x})^2 + 2\sqrt{x} + 2}{-(6 - \sqrt{x} - (\sqrt{x})^2)} \ge 0
(x)2+2x+26−x−(x)2≤0\frac{(\sqrt{x})^2 + 2\sqrt{x} + 2}{6 - \sqrt{x} - (\sqrt{x})^2} \le 0

4. Solve the third inequality using the substitution $y = \sqrt{x}$ and the previous result.

Letting y=xy = \sqrt{x}, the inequality becomes y2+2y+26−y−y2≤0\frac{y^2 + 2y + 2}{6 - y - y^2} \le 0.
From the previous part, the solution is y<−3 or y>2y < -3 \text{ or } y > 2.
Substituting back y=xy = \sqrt{x}, we have:
x<−3 or x>2\sqrt{x} < -3 \text{ or } \sqrt{x} > 2
Since x≥0\sqrt{x} \ge 0 for all real x≥0x \ge 0, x<−3\sqrt{x} < -3 yields no real solution.
For x>2\sqrt{x} > 2, squaring both sides (since both are positive) gives x>4x > 4.

Answer

Part 2: x<−3 or x>2x < -3 \text{ or } x > 2
Part 3: x>4x > 4

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