Question

Given the equation \( 5\sin(x) - 12\cos(x) = 0 \), use the auxiliary angle method to find all possible values of x within one period.

Ask by Olson Chadwick. in Nigeria
Feb 25,2025

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Answer

The solutions within one period are \( x = \arctan\left(\frac{12}{5}\right) \) and \( x = \arctan\left(\frac{12}{5}\right) + \pi \).

Solution

We start with the equation   5 sin x – 12 cos x = 0. The auxiliary angle method involves writing the left-hand side in the form   R sin(x – φ), where   R = √(5² + 12²) = 13, and φ is chosen so that   cos φ = 5/13  and  sin φ = 12/13. Notice that   sin(x – φ) = sin x cos φ – cos x sin φ = (5/13) sin x – (12/13) cos x. Multiplying through by 13 yields   13 sin(x – φ) = 5 sin x – 12 cos x. Thus, our equation becomes   13 sin(x – φ) = 0  ⟹  sin(x – φ) = 0. Since sinθ = 0 when θ = nπ (where n is an integer), we have   x – φ = nπ  ⟹  x = φ + nπ. Within one period (0 ≤ x < 2π), take n = 0 and n = 1 to obtain   x = φ = arctan(12/5),   x = φ + π = arctan(12/5) + π. Thus, the solutions within one period are   x = arctan(12/5)  and  x = arctan(12/5) + π. You could also have solved the original equation by isolating tan x:   5 sin x = 12 cos x  ⟹  tan x = 12/5  ⟹  x = arctan(12/5) + nπ, which yields the same answers.

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To solve the equation \( 5\sin(x) - 12\cos(x) = 0 \) using the auxiliary angle method, we start by rearranging it to isolate the sine and cosine terms: \[ 5\sin(x) = 12\cos(x) \] Dividing both sides by \( \cos(x) \) (assuming \( \cos(x) \neq 0 \)), we obtain: \[ \tan(x) = \frac{12}{5} \] Next, to find the angle whose tangent is \( \frac{12}{5} \), we can let \( r = \sqrt{5^2 + 12^2} = \sqrt{25 + 144} = \sqrt{169} = 13 \). Now we set values for \( \sin \) and \( \cos \): \[ \sin(x) = \frac{12}{13}, \quad \cos(x) = \frac{5}{13} \] Using the identity \( \sin(x) = R\sin(x + \phi) \) where \( R = 13 \) and \( \tan(\phi) = \frac{12}{5} \): Now we find \( \phi \) using the tangent: \[ \phi = \tan^{-1}\left(\frac{12}{5}\right) \] Now, rewrite the original equation in the form: \[ 13\sin(x + \phi) = 0 \implies \sin(x + \phi) = 0 \] The solutions for \( \sin(x + \phi) = 0 \) are given by: \[ x + \phi = n\pi \] Thus, \[ x = n\pi - \phi \] Considering \( n = 0 \) and \( n = 1 \) for one period \( [0, 2\pi) \): 1. \( x = -\phi \) (not within \([0, 2\pi)\)) 2. \( x = \pi - \phi \) So we compute \( \phi \): \[ \phi = \tan^{-1}\left(\frac{12}{5}\right) \approx 1.176 \] Calculating \( \pi - \phi \): \[ \pi - \phi \approx 3.142 - 1.176 \approx 1.966 \] To summarize, the solutions in the interval \( [0, 2\pi) \) are: \[ x \approx 1.966 \quad \text{and} \quad x = \pi + (n\pi - \phi) \text{ for } n = 1 \rightarrow 4.142 \] Thus the possible values of \( x \) are approximately: \[ 1.966 \text{ and } 4.142 \]

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