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Determine general solution \( 1+4 \sin ^{2} x-5 \sin x+\cos 2 x=0 \)

Ask by Chang Cox. in South Africa
Feb 26,2025

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Answer

The general solution to the equation \(1+4 \sin ^{2} x-5 \sin x+\cos 2x=0\) is: \[x = \frac{\pi}{6} + 2k\pi \quad \text{or} \quad x = \frac{5\pi}{6} + 2k\pi\] where \(k\) is any integer.

Solution

Solve the equation \( 1+4 \sin ^{2}(x)-5 \sin (x)+\cos (2x)=0 \). Solve the equation by following steps: - step0: Solve for \(x\): \(1+4\sin^{2}\left(x\right)-5\sin\left(x\right)+\cos\left(2x\right)=0\) - step1: Rewrite the expression: \(1+4\sin^{2}\left(x\right)-5\sin\left(x\right)+2\cos^{2}\left(x\right)-1=0\) - step2: Expand the expression: \(4\sin^{2}\left(x\right)-5\sin\left(x\right)+2\cos^{2}\left(x\right)=0\) - step3: Rewrite the expression: \(2\sin^{2}\left(x\right)-5\sin\left(x\right)+2=0\) - step4: Factor the expression: \(\left(\sin\left(x\right)-2\right)\left(2\sin\left(x\right)-1\right)=0\) - step5: Separate into possible cases: \(\begin{align}&\sin\left(x\right)-2=0\\&2\sin\left(x\right)-1=0\end{align}\) - step6: Solve the equation: \(\begin{align}&x \notin \mathbb{R}\\&x=\left\{ \begin{array}{l}\frac{\pi }{6}+2k\pi \\\frac{5\pi }{6}+2k\pi \end{array}\right.,k \in \mathbb{Z}\end{align}\) - step7: Find the union: \(x=\left\{ \begin{array}{l}\frac{\pi }{6}+2k\pi \\\frac{5\pi }{6}+2k\pi \end{array}\right.,k \in \mathbb{Z}\) The general solution to the equation \(1+4 \sin ^{2} x-5 \sin x+\cos 2x=0\) is given by: \[x = \left\{ \begin{array}{l} \frac{\pi}{6} + 2k\pi \\ \frac{5\pi}{6} + 2k\pi \end{array} \right., k \in \mathbb{Z}\] where \(k\) is an integer.

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To solve the equation \(1 + 4 \sin^2 x - 5 \sin x + \cos 2x = 0\), we first utilize the identity \( \cos 2x = 1 - 2\sin^2 x \). Substituting this identity into the equation gives: \[ 1 + 4 \sin^2 x - 5 \sin x + (1 - 2\sin^2 x) = 0. \] Simplifying yields: \[ 2 + 2\sin^2 x - 5 \sin x = 0 \quad \Rightarrow \quad 2\sin^2 x - 5\sin x + 2 = 0. \] Next, we can apply the quadratic formula where \( a = 2\), \( b = -5\), and \( c = 2\): \[ \sin x = \frac{-(-5) \pm \sqrt{(-5)^2 - 4(2)(2)}}{2(2)} = \frac{5 \pm \sqrt{25 - 16}}{4} = \frac{5 \pm 3}{4}. \] This results in two potential solutions for \( \sin x \): \[ \sin x = \frac{8}{4} = 2 \quad \text{(not valid since } |\sin x| \leq 1\text{)}, \] \[ \sin x = \frac{2}{4} = \frac{1}{2}. \] The valid solution where \( \sin x = \frac{1}{2} \) occurs at: \[ x = \frac{\pi}{6} + 2k\pi \quad \text{and} \quad x = \frac{5\pi}{6} + 2k\pi, \quad k \in \mathbb{Z}. \] So, the general solution is: \[ x = \frac{\pi}{6} + 2k\pi \quad \text{and} \quad x = \frac{5\pi}{6} + 2k\pi, \quad k \in \mathbb{Z}. \] Happy angle hunting! ✨

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