The problem consists of four parts: 1. Verify the equality $\sqrt{3 + 2\sqrt{2}} = 1 + \sqrt{2}$.

AlgebraRadicalsQuadratic EquationsQuadratic InequalitiesTrigonometryTrigonometric EquationsTrigonometric Inequalities
2025/4/14

1. Problem Description

The problem consists of four parts:

1. Verify the equality $\sqrt{3 + 2\sqrt{2}} = 1 + \sqrt{2}$.

2. Solve the quadratic equation $2x^2 + (1 - \sqrt{2})x - \frac{\sqrt{2}}{2} = 0$ and the inequality $2x^2 + (1 - \sqrt{2})x - \frac{\sqrt{2}}{2} > 0$ in the real numbers.

3. Find the solutions of the trigonometric equation $2\cos^2x + (1 - \sqrt{2})\cos x - \frac{\sqrt{2}}{2} = 0$. Represent the solutions on the unit circle.

4. Find the solutions in $[0, 2\pi]$ of the inequality $2\cos^2x + (1 - \sqrt{2})\cos x - \frac{\sqrt{2}}{2} > 0$. Represent the solutions on the unit circle.

2. Solution Steps

1. Verify the equality:

(1+2)2=1+22+2=3+22(1 + \sqrt{2})^2 = 1 + 2\sqrt{2} + 2 = 3 + 2\sqrt{2}. Thus 3+22=1+2\sqrt{3 + 2\sqrt{2}} = 1 + \sqrt{2}.

2. Solve the quadratic equation and inequality:

The equation is 2x2+(12)x22=02x^2 + (1 - \sqrt{2})x - \frac{\sqrt{2}}{2} = 0.
We can multiply by 2 to get 4x2+2(12)x2=04x^2 + 2(1 - \sqrt{2})x - \sqrt{2} = 0.
The discriminant is Δ=(12)24(2)(22)=122+2+42=3+22=(1+2)2\Delta = (1 - \sqrt{2})^2 - 4(2)(-\frac{\sqrt{2}}{2}) = 1 - 2\sqrt{2} + 2 + 4\sqrt{2} = 3 + 2\sqrt{2} = (1 + \sqrt{2})^2.
Thus the solutions are x1,2=(12)±(1+2)24=1+2±(1+2)4x_{1,2} = \frac{-(1 - \sqrt{2}) \pm \sqrt{(1 + \sqrt{2})^2}}{4} = \frac{-1 + \sqrt{2} \pm (1 + \sqrt{2})}{4}.
x1=1+2+1+24=224=22x_1 = \frac{-1 + \sqrt{2} + 1 + \sqrt{2}}{4} = \frac{2\sqrt{2}}{4} = \frac{\sqrt{2}}{2}.
x2=1+2124=24=12x_2 = \frac{-1 + \sqrt{2} - 1 - \sqrt{2}}{4} = \frac{-2}{4} = -\frac{1}{2}.
Now, consider the inequality 2x2+(12)x22>02x^2 + (1 - \sqrt{2})x - \frac{\sqrt{2}}{2} > 0.
Since the leading coefficient is positive, the parabola opens upward. The inequality is satisfied when x<12x < -\frac{1}{2} or x>22x > \frac{\sqrt{2}}{2}.

3. Solve the trigonometric equation:

Let y=cosxy = \cos x. Then the equation becomes 2y2+(12)y22=02y^2 + (1 - \sqrt{2})y - \frac{\sqrt{2}}{2} = 0.
The solutions are y1=22y_1 = \frac{\sqrt{2}}{2} and y2=12y_2 = -\frac{1}{2}.
So cosx=22\cos x = \frac{\sqrt{2}}{2} or cosx=12\cos x = -\frac{1}{2}.
If cosx=22\cos x = \frac{\sqrt{2}}{2}, then x=±π4+2kπx = \pm \frac{\pi}{4} + 2k\pi, kZk \in \mathbb{Z}.
If cosx=12\cos x = -\frac{1}{2}, then x=±2π3+2kπx = \pm \frac{2\pi}{3} + 2k\pi, kZk \in \mathbb{Z}.
The solutions are x=π4+2kπ,π4+2kπ,2π3+2kπ,2π3+2kπx = \frac{\pi}{4} + 2k\pi, -\frac{\pi}{4} + 2k\pi, \frac{2\pi}{3} + 2k\pi, -\frac{2\pi}{3} + 2k\pi for kZk \in \mathbb{Z}.

4. Solve the trigonometric inequality in $[0, 2\pi]$:

2cos2x+(12)cosx22>02\cos^2x + (1 - \sqrt{2})\cos x - \frac{\sqrt{2}}{2} > 0.
Let y=cosxy = \cos x. Then we have 2y2+(12)y22>02y^2 + (1 - \sqrt{2})y - \frac{\sqrt{2}}{2} > 0. The solutions are y<12y < -\frac{1}{2} or y>22y > \frac{\sqrt{2}}{2}.
So cosx<12\cos x < -\frac{1}{2} or cosx>22\cos x > \frac{\sqrt{2}}{2}.
cosx>22\cos x > \frac{\sqrt{2}}{2} when x[0,π4)(7π4,2π]x \in [0, \frac{\pi}{4}) \cup (\frac{7\pi}{4}, 2\pi].
cosx<12\cos x < -\frac{1}{2} when x(2π3,4π3)x \in (\frac{2\pi}{3}, \frac{4\pi}{3}).
Thus, the solution is x[0,π4)(2π3,4π3)(7π4,2π]x \in [0, \frac{\pi}{4}) \cup (\frac{2\pi}{3}, \frac{4\pi}{3}) \cup (\frac{7\pi}{4}, 2\pi].

3. Final Answer

1. $\sqrt{3 + 2\sqrt{2}} = 1 + \sqrt{2}$

2. The solutions to the equation are $x = \frac{\sqrt{2}}{2}$ and $x = -\frac{1}{2}$. The solution to the inequality is $x < -\frac{1}{2}$ or $x > \frac{\sqrt{2}}{2}$.

3. The solutions to the trigonometric equation are $x = \pm \frac{\pi}{4} + 2k\pi, \pm \frac{2\pi}{3} + 2k\pi$ for $k \in \mathbb{Z}$.

4. The solution to the trigonometric inequality in $[0, 2\pi]$ is $x \in [0, \frac{\pi}{4}) \cup (\frac{2\pi}{3}, \frac{4\pi}{3}) \cup (\frac{7\pi}{4}, 2\pi]$.