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Express 4 cos θ – 3 sin θ in the form R sin(θ – α), where R > 0 and 0° < α < 360°, giving α to the nearest degree. Solve the equation 4 cos θ – 3 sin θ = 3, for 0° ≤ θ ≤ 360°.

Mathematics

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Answers

2 parts
a

5sin⁡(θ−233∘)5 \sin(\theta - 233^\circ)

b

θ=16.1∘,269.9∘\theta = 16.1^\circ, 269.9^\circ

Step-by-step solution

8 steps

Part a· 3 steps

We want to express 4cos⁡θ−3sin⁡θ4 \cos \theta - 3 \sin \theta in the form Rsin⁡(θ−α)R \sin(\theta - \alpha).

First, expand Rsin⁡(θ−α)R \sin(\theta - \alpha) using the compound angle formula:

Rsin⁡(θ−α)=R(sin⁡θcos⁡α−cos⁡θsin⁡α)R \sin(\theta - \alpha) = R(\sin \theta \cos \alpha - \cos \theta \sin \alpha)

Rearrange the given expression to match the terms:

4cos⁡θ−3sin⁡θ=−3sin⁡θ+4cos⁡θ4 \cos \theta - 3 \sin \theta = -3 \sin \theta + 4 \cos \theta

Comparing coefficients:

−3=Rcos⁡α(1)-3 = R \cos \alpha \quad \text{(1)} 4=−Rsin⁡α  ⟹  Rsin⁡α=−4(2)4 = -R \sin \alpha \implies R \sin \alpha = -4 \quad \text{(2)}
  1. Find RR.

    Square equations (1) and (2) and add them:

    (Rcos⁡α)2+(Rsin⁡α)2=(−3)2+(−4)2(R \cos \alpha)^2 + (R \sin \alpha)^2 = (-3)^2 + (-4)^2 R2(cos⁡2α+sin⁡2α)=9+16R^2 (\cos^2 \alpha + \sin^2 \alpha) = 9 + 16 R2(1)=25R^2 (1) = 25

    Since R>0R > 0,

    R=25=5R = \sqrt{25} = 5
  2. Find α\alpha.

    Divide equation (2) by equation (1):

    Rsin⁡αRcos⁡α=−4−3\frac{R \sin \alpha}{R \cos \alpha} = \frac{-4}{-3} tan⁡α=43\tan \alpha = \frac{4}{3}

    Since Rcos⁡α=−3R \cos \alpha = -3 (negative) and Rsin⁡α=−4R \sin \alpha = -4 (negative), α\alpha must be in the third quadrant.
    The reference angle is arctan⁡(43)≈53.13∘\arctan\left(\frac{4}{3}\right) \approx 53.13^\circ.
    Therefore, α=180∘+53.13∘=233.13∘\alpha = 180^\circ + 53.13^\circ = 233.13^\circ.
    Rounding to the nearest degree, α=233∘\alpha = 233^\circ.

  3. Write the expression in the required form.

    4cos⁡θ−3sin⁡θ=5sin⁡(θ−233∘)4 \cos \theta - 3 \sin \theta = 5 \sin(\theta - 233^\circ)

    The final answer is 5sin⁡(θ−233∘)\boxed{5 \sin(\theta - 233^\circ)}.

Part b

θ=16.1∘,269.9∘\theta = 16.1^\circ, 269.9^\circ

We need to solve the equation 4cos⁡θ−3sin⁡θ=34 \cos \theta - 3 \sin \theta = 3 for 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ.

From part (a), we know that 4cos⁡θ−3sin⁡θ=5sin⁡(θ−233∘)4 \cos \theta - 3 \sin \theta = 5 \sin(\theta - 233^\circ).
So, the equation becomes:

5sin⁡(θ−233∘)=35 \sin(\theta - 233^\circ) = 3
  1. Isolate the sine term.

    sin⁡(θ−233∘)=35=0.6\sin(\theta - 233^\circ) = \frac{3}{5} = 0.6
  2. Let ϕ=θ−233∘\phi = \theta - 233^\circ.

    Determine the range for ϕ\phi.
    Given 0∘≤θ≤360∘0^\circ \le \theta \le 360^\circ:

    0∘−233∘≤θ−233∘≤360∘−233∘0^\circ - 233^\circ \le \theta - 233^\circ \le 360^\circ - 233^\circ −233∘≤ϕ≤127∘-233^\circ \le \phi \le 127^\circ
  3. Find the principal value for ϕ\phi.

    ϕ1=arcsin⁡(0.6)≈36.87∘\phi_1 = \arcsin(0.6) \approx 36.87^\circ

    This value is within the range [−233∘,127∘][-233^\circ, 127^\circ].

  4. Find other possible values for ϕ\phi within the range.

    Since sin⁡ϕ\sin \phi is positive, ϕ\phi can be in the first or second quadrant.
    The second general solution is 180∘−ϕ1180^\circ - \phi_1.

    ϕ2=180∘−36.87∘=143.13∘\phi_2 = 180^\circ - 36.87^\circ = 143.13^\circ

    This value is not within the range [−233∘,127∘][-233^\circ, 127^\circ] as 143.13∘>127∘143.13^\circ > 127^\circ.

    We also need to consider solutions by subtracting 360∘360^\circ.
    For ϕ1\phi_1: 36.87∘−360∘=−323.13∘36.87^\circ - 360^\circ = -323.13^\circ. This is not within the range as −323.13∘<−233∘-323.13^\circ < -233^\circ.
    For ϕ2\phi_2: 143.13∘−360∘=−216.87∘143.13^\circ - 360^\circ = -216.87^\circ. This value is within the range [−233∘,127∘][-233^\circ, 127^\circ].

    So, the valid values for ϕ\phi are approximately 36.87∘36.87^\circ and −216.87∘-216.87^\circ.

  5. Solve for θ\theta using θ=ϕ+233∘\theta = \phi + 233^\circ.

    For ϕ=36.87∘\phi = 36.87^\circ:

    θ1=36.87∘+233∘=269.87∘\theta_1 = 36.87^\circ + 233^\circ = 269.87^\circ

    For ϕ=−216.87∘\phi = -216.87^\circ:

    θ2=−216.87∘+233∘=16.13∘\theta_2 = -216.87^\circ + 233^\circ = 16.13^\circ

    Rounding to one decimal place:

    θ1≈269.9∘\theta_1 \approx 269.9^\circ θ2≈16.1∘\theta_2 \approx 16.1^\circ

    The final answers are θ=16.1∘,269.9∘\boxed{\theta = 16.1^\circ, 269.9^\circ}.

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