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Solve the following equations for 0≤0≤360°: i) 2 Sec20+tan 0−3=0 ii) Cos20 + Sin 0+1=0

Mathematics

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Answers

2 parts
i

26.57∘,135∘,206.57∘,315∘26.57^\circ, 135^\circ, 206.57^\circ, 315^\circ

ii

270∘270^\circ

Step-by-step solution

7 steps

Part 1

Part i

26.57∘,135∘,206.57∘,315∘26.57^\circ, 135^\circ, 206.57^\circ, 315^\circ

Solve 2sec⁡2θ+tan⁡θ−3=02 \sec^2 \theta + \tan \theta - 3 = 0 for 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

  1. Use the trigonometric identity sec⁡2θ=1+tan⁡2θ\sec^2 \theta = 1 + \tan^2 \theta to rewrite the equation in terms of tan⁡θ\tan \theta.

    2(1+tan⁡2θ)+tan⁡θ−3=02(1 + \tan^2 \theta) + \tan \theta - 3 = 0 2+2tan⁡2θ+tan⁡θ−3=02 + 2\tan^2 \theta + \tan \theta - 3 = 0 2tan⁡2θ+tan⁡θ−1=02\tan^2 \theta + \tan \theta - 1 = 0
  2. Let y=tan⁡θy = \tan \theta to form a quadratic equation and solve for yy.

    2y2+y−1=02y^2 + y - 1 = 0

    Factor the quadratic equation:

    (2y−1)(y+1)=0(2y - 1)(y + 1) = 0

    This gives two possible values for yy:

    2y−1=0  ⟹  y=122y - 1 = 0 \implies y = \frac{1}{2} y+1=0  ⟹  y=−1y + 1 = 0 \implies y = -1

    So, tan⁡θ=12\tan \theta = \frac{1}{2} or tan⁡θ=−1\tan \theta = -1.

  3. Find the values of θ\theta for tan⁡θ=12\tan \theta = \frac{1}{2} in the range 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

    Since tan⁡θ\tan \theta is positive, θ\theta is in Quadrant I or Quadrant III.
    The reference angle is α=arctan⁡(12)≈26.565∘\alpha = \arctan\left(\frac{1}{2}\right) \approx 26.565^\circ.
    In Quadrant I: θ1=α≈26.57∘\theta_1 = \alpha \approx 26.57^\circ.
    In Quadrant III: θ2=180∘+α≈180∘+26.565∘=206.565∘≈206.57∘\theta_2 = 180^\circ + \alpha \approx 180^\circ + 26.565^\circ = 206.565^\circ \approx 206.57^\circ.

  4. Find the values of θ\theta for tan⁡θ=−1\tan \theta = -1 in the range 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

    Since tan⁡θ\tan \theta is negative, θ\theta is in Quadrant II or Quadrant IV.
    The reference angle is α=arctan⁡(1)=45∘\alpha = \arctan(1) = 45^\circ.
    In Quadrant II: θ3=180∘−α=180∘−45∘=135∘\theta_3 = 180^\circ - \alpha = 180^\circ - 45^\circ = 135^\circ.
    In Quadrant IV: θ4=360∘−α=360∘−45∘=315∘\theta_4 = 360^\circ - \alpha = 360^\circ - 45^\circ = 315^\circ.
    The solutions are 26.57∘,135∘,206.57∘,315∘\boxed{26.57^\circ, 135^\circ, 206.57^\circ, 315^\circ}.

Part ii

270∘270^\circ

Solve cos⁡2θ+sin⁡θ+1=0\cos^2 \theta + \sin \theta + 1 = 0 for 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

  1. Use the trigonometric identity cos⁡2θ=1−sin⁡2θ\cos^2 \theta = 1 - \sin^2 \theta to rewrite the equation in terms of sin⁡θ\sin \theta.

    (1−sin⁡2θ)+sin⁡θ+1=0(1 - \sin^2 \theta) + \sin \theta + 1 = 0 2+sin⁡θ−sin⁡2θ=02 + \sin \theta - \sin^2 \theta = 0

    Multiply by −1-1 to rearrange into standard quadratic form:

    sin⁡2θ−sin⁡θ−2=0\sin^2 \theta - \sin \theta - 2 = 0
  2. Let y=sin⁡θy = \sin \theta to form a quadratic equation and solve for yy.

    y2−y−2=0y^2 - y - 2 = 0

    Factor the quadratic equation:

    (y−2)(y+1)=0(y - 2)(y + 1) = 0

    This gives two possible values for yy:

    y−2=0  ⟹  y=2y - 2 = 0 \implies y = 2 y+1=0  ⟹  y=−1y + 1 = 0 \implies y = -1

    So, sin⁡θ=2\sin \theta = 2 or sin⁡θ=−1\sin \theta = -1.

  3. Find the values of θ\theta for each case in the range 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

    For sin⁡θ=2\sin \theta = 2: There is no solution because the range of the sine function is [−1,1][-1, 1].
    For sin⁡θ=−1\sin \theta = -1: This occurs at θ=270∘\theta = 270^\circ.
    The solution is 270∘\boxed{270^\circ}.

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