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Determine the general solution of: \[ \sin 2 x=\cos (x+60) \]

Ask by Cross Mejia. in South Africa
Mar 11,2025

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Answer

The general solution is: \[ x = \frac{41\pi -120}{6} + \frac{2k\pi}{3} \] and \[ x = \frac{\pi +120}{2} + 2k\pi \] where \(k\) is any integer.

Solution

Solve the equation \( \sin(2x)=\cos(x+60) \). Solve the equation by following steps: - step0: Solve for \(x\): \(\sin\left(2x\right)=\cos\left(x+60\right)\) - step1: Move the expression to the left side: \(\sin\left(2x\right)-\cos\left(x+60\right)=0\) - step2: Transform the expression: \(-2\sin\left(\frac{\pi +120-2x}{4}\right)\sin\left(\frac{\pi -120-6x}{4}\right)=0\) - step3: Elimination the left coefficient: \(\sin\left(\frac{\pi +120-2x}{4}\right)\sin\left(\frac{\pi -120-6x}{4}\right)=0\) - step4: Separate into possible cases: \(\begin{align}&\sin\left(\frac{\pi +120-2x}{4}\right)=0\\&\sin\left(\frac{\pi -120-6x}{4}\right)=0\end{align}\) - step5: Solve the equation: \(\begin{align}&x=\frac{\pi +120}{2}+2k\pi ,k \in \mathbb{Z}\\&x=\frac{41\pi -120}{6}+\frac{2k\pi }{3},k \in \mathbb{Z}\end{align}\) - step6: Find the union: \(x=\left\{ \begin{array}{l}\frac{41\pi -120}{6}+\frac{2k\pi }{3}\\\frac{\pi +120}{2}+2k\pi \end{array}\right.,k \in \mathbb{Z}\) The general solution of the equation \(\sin 2x = \cos (x+60)\) is given by: \[ x = \left\{ \begin{array}{l} \frac{41\pi -120}{6} + \frac{2k\pi}{3} \\ \frac{\pi +120}{2} + 2k\pi \end{array} \right., k \in \mathbb{Z} \] where \(k\) is an integer.

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Bonus Knowledge

To solve the equation \(\sin 2x = \cos(x + 60^\circ)\), we can use the co-function identity, which states that \(\sin \theta = \cos(90^\circ - \theta)\). First, we rewrite \(\cos(x + 60^\circ)\) in terms of sine: \[ \sin 2x = \cos(x + 60^\circ) \implies \sin 2x = \sin(90^\circ - (x + 60^\circ)) \] Thus, we have: \[ \sin 2x = \sin(30^\circ - x) \] Now, we can set up the general cases for solving \(\sin A = \sin B\): 1. \(2x = 30^\circ - x + 360^\circ n\) 2. \(2x = 180^\circ - (30^\circ - x) + 360^\circ n\) where \(n\) is any integer. **For the first case:** \[ 2x = 30^\circ - x + 360^\circ n \implies 3x = 30^\circ + 360^\circ n \implies x = 10^\circ + 120^\circ n \] **For the second case:** \[ 2x = 180^\circ - (30^\circ - x) + 360^\circ n \implies 2x = 180^\circ - 30^\circ + x + 360^\circ n \] \[ 2x = 150^\circ + x + 360^\circ n \implies x = 150^\circ + 360^\circ n \] Now we compile our solutions: 1. \(x = 10^\circ + 120^\circ n\) 2. \(x = 150^\circ + 360^\circ n\) Combining both solutions, the general solutions of the given equation \(\sin 2x = \cos(x + 60^\circ)\) are: \[ x = 10^\circ + 120^\circ n \quad \text{and} \quad x = 150^\circ + 360^\circ n \] where \(n\) is any integer.

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