The problem is to solve the trigonometric equation $2sin^2(x) - \sqrt{2}cos(x) = 0$.

AlgebraTrigonometryTrigonometric EquationsQuadratic EquationsTrigonometric IdentitiesSolving Equations
2025/5/22

1. Problem Description

The problem is to solve the trigonometric equation 2sin2(x)2cos(x)=02sin^2(x) - \sqrt{2}cos(x) = 0.

2. Solution Steps

We need to find the values of xx that satisfy the equation 2sin2(x)2cos(x)=02sin^2(x) - \sqrt{2}cos(x) = 0.
We can use the trigonometric identity sin2(x)+cos2(x)=1sin^2(x) + cos^2(x) = 1 to express sin2(x)sin^2(x) in terms of cos2(x)cos^2(x).
sin2(x)=1cos2(x)sin^2(x) = 1 - cos^2(x)
Substituting this into the equation, we get:
2(1cos2(x))2cos(x)=02(1 - cos^2(x)) - \sqrt{2}cos(x) = 0
22cos2(x)2cos(x)=02 - 2cos^2(x) - \sqrt{2}cos(x) = 0
Rearranging the terms, we get a quadratic equation in terms of cos(x)cos(x):
2cos2(x)+2cos(x)2=02cos^2(x) + \sqrt{2}cos(x) - 2 = 0
Let y=cos(x)y = cos(x). Then the equation becomes:
2y2+2y2=02y^2 + \sqrt{2}y - 2 = 0
We can solve for yy using the quadratic formula:
y=b±b24ac2ay = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
where a=2a=2, b=2b=\sqrt{2}, and c=2c=-2.
y=2±(2)24(2)(2)2(2)y = \frac{-\sqrt{2} \pm \sqrt{(\sqrt{2})^2 - 4(2)(-2)}}{2(2)}
y=2±2+164y = \frac{-\sqrt{2} \pm \sqrt{2 + 16}}{4}
y=2±184y = \frac{-\sqrt{2} \pm \sqrt{18}}{4}
y=2±324y = \frac{-\sqrt{2} \pm 3\sqrt{2}}{4}
So we have two possible values for yy:
y1=2+324=224=22y_1 = \frac{-\sqrt{2} + 3\sqrt{2}}{4} = \frac{2\sqrt{2}}{4} = \frac{\sqrt{2}}{2}
y2=2324=424=2y_2 = \frac{-\sqrt{2} - 3\sqrt{2}}{4} = \frac{-4\sqrt{2}}{4} = -\sqrt{2}
Since y=cos(x)y = cos(x), we have cos(x)=22cos(x) = \frac{\sqrt{2}}{2} or cos(x)=2cos(x) = -\sqrt{2}.
Since 1cos(x)1-1 \leq cos(x) \leq 1, the second solution cos(x)=2cos(x) = -\sqrt{2} is not possible as 21.414-\sqrt{2} \approx -1.414.
Therefore, cos(x)=22cos(x) = \frac{\sqrt{2}}{2}.
The values of xx for which cos(x)=22cos(x) = \frac{\sqrt{2}}{2} are x=π4+2nπx = \frac{\pi}{4} + 2n\pi and x=π4+2nπx = -\frac{\pi}{4} + 2n\pi, where nn is an integer. We can also write x=7π4+2nπx = \frac{7\pi}{4} + 2n\pi.
Combining the two solutions gives x=±π4+2nπx = \pm \frac{\pi}{4} + 2n\pi, where nn is an integer.

3. Final Answer

x=π4+2nπx = \frac{\pi}{4} + 2n\pi and x=7π4+2nπx = \frac{7\pi}{4} + 2n\pi, where nn is an integer. Or x=±π4+2nπx = \pm \frac{\pi}{4} + 2n\pi, where nn is an integer.

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