Solve the equation sin2x = cotx

Mathematics

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Step-by-step solution

7 steps
  1. Rewrite the trigonometric functions in terms of sin⁡(x)\sin(x) and cos⁡(x)\cos(x).

    We know that sin⁡(2x)=2sin⁡(x)cos⁡(x)\sin(2x) = 2\sin(x)\cos(x) and cot⁡(x)=cos⁡(x)sin⁡(x)\cot(x) = \frac{\cos(x)}{\sin(x)}.
    Substitute these into the given equation:

    2sin⁡(x)cos⁡(x)=cos⁡(x)sin⁡(x)2\sin(x)\cos(x) = \frac{\cos(x)}{\sin(x)}

    Note that for cot⁡(x)\cot(x) to be defined, sin⁡(x)≠0\sin(x) \neq 0. This means x≠nπx \neq n\pi for any integer nn.

  2. Rearrange the equation to solve for xx.

    Multiply both sides by sin⁡(x)\sin(x) (which is non-zero based on the restriction):

    2sin⁡2(x)cos⁡(x)=cos⁡(x)2\sin^2(x)\cos(x) = \cos(x)

    Move all terms to one side:

    2sin⁡2(x)cos⁡(x)−cos⁡(x)=02\sin^2(x)\cos(x) - \cos(x) = 0
  3. Factor out the common term cos⁡(x)\cos(x).

    cos⁡(x)(2sin⁡2(x)−1)=0\cos(x)(2\sin^2(x) - 1) = 0
  4. Set each factor equal to zero and solve for xx.

    This gives two separate cases:
    Case 1: cos⁡(x)=0\cos(x) = 0
    Case 2: 2sin⁡2(x)−1=02\sin^2(x) - 1 = 0

  5. Solve Case 1.

    If cos⁡(x)=0\cos(x) = 0, then xx is an odd multiple of π2\frac{\pi}{2}.

    x=π2+nπx = \frac{\pi}{2} + n\pi

    where nn is an integer.

  6. Solve Case 2.

    If 2sin⁡2(x)−1=02\sin^2(x) - 1 = 0, then:

    2sin⁡2(x)=12\sin^2(x) = 1 sin⁡2(x)=12\sin^2(x) = \frac{1}{2}

    We can use the identity cos⁡(2x)=1−2sin⁡2(x)\cos(2x) = 1 - 2\sin^2(x).
    So, 2sin⁡2(x)−1=−(1−2sin⁡2(x))=−cos⁡(2x)2\sin^2(x) - 1 = -(1 - 2\sin^2(x)) = -\cos(2x).
    Therefore, the equation becomes −cos⁡(2x)=0-\cos(2x) = 0, or cos⁡(2x)=0\cos(2x) = 0.
    If cos⁡(2x)=0\cos(2x) = 0, then 2x2x is an odd multiple of π2\frac{\pi}{2}.

    2x=π2+nπ2x = \frac{\pi}{2} + n\pi

    Divide by 2:

    x=π4+nπ2x = \frac{\pi}{4} + \frac{n\pi}{2}

    where nn is an integer.

  7. Verify solutions against the restriction sin⁡(x)≠0\sin(x) \neq 0.

    For x=π2+nπx = \frac{\pi}{2} + n\pi, sin⁡(x)=±1\sin(x) = \pm 1, which is not zero. These solutions are valid.
    For x=π4+nπ2x = \frac{\pi}{4} + \frac{n\pi}{2}, sin⁡(x)=±22\sin(x) = \pm \frac{\sqrt{2}}{2}, which is not zero. These solutions are valid.

    The general solutions are the union of the solutions from Case 1 and Case 2.
    The solutions are:

    x=π2+nπorx=π4+nπ2x = \frac{\pi}{2} + n\pi \quad \text{or} \quad x = \frac{\pi}{4} + \frac{n\pi}{2}

    where n∈Zn \in \mathbb{Z}.

    The final answer is x=π2+nπ or x=π4+nπ2, where n∈Z\boxed{x = \frac{\pi}{2} + n\pi \text{ or } x = \frac{\pi}{4} + \frac{n\pi}{2}, \text{ where } n \in \mathbb{Z}}.

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