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Find the exact value of each of the remaining trigonometric functions of \( \theta \) \[ \cos \theta=\frac{4}{5}, 270^{\circ}<\theta<360^{\circ} \]

Ask by King Park. in the United States
Mar 10,2025

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\[ \sin \theta = -\frac{3}{5}, \quad \tan \theta = -\frac{3}{4}, \quad \sec \theta = \frac{5}{4}, \quad \csc \theta = -\frac{5}{3}, \quad \cot \theta = -\frac{4}{3} \]

Solution

1. Since \( \cos \theta = \frac{4}{5} \) and \( 270^\circ < \theta < 360^\circ \) (Quadrant IV), we know: - \( \cos \theta > 0 \) - \( \sin \theta < 0 \) 2. Use the Pythagorean identity: \[ \sin^2 \theta + \cos^2 \theta = 1 \] Substitute \( \cos \theta = \frac{4}{5} \): \[ \sin^2 \theta + \left(\frac{4}{5}\right)^2 = 1 \quad \Rightarrow \quad \sin^2 \theta + \frac{16}{25} = 1 \] Solve for \( \sin^2 \theta \): \[ \sin^2 \theta = 1 - \frac{16}{25} = \frac{25}{25} - \frac{16}{25} = \frac{9}{25} \] Taking the square root (and choosing the negative value in Quadrant IV): \[ \sin \theta = -\frac{3}{5} \] 3. Compute the remaining trigonometric functions: - Tangent: \[ \tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{-\frac{3}{5}}{\frac{4}{5}} = -\frac{3}{4} \] - Secant (reciprocal of cosine): \[ \sec \theta = \frac{1}{\cos \theta} = \frac{1}{\frac{4}{5}} = \frac{5}{4} \] - Cosecant (reciprocal of sine): \[ \csc \theta = \frac{1}{\sin \theta} = \frac{1}{-\frac{3}{5}} = -\frac{5}{3} \] - Cotangent (reciprocal of tangent): \[ \cot \theta = \frac{1}{\tan \theta} = \frac{1}{-\frac{3}{4}} = -\frac{4}{3} \] 4. The exact values of the remaining trigonometric functions are: - \(\sin \theta = -\frac{3}{5}\) - \(\tan \theta = -\frac{3}{4}\) - \(\sec \theta = \frac{5}{4}\) - \(\csc \theta = -\frac{5}{3}\) - \(\cot \theta = -\frac{4}{3}\)

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To find the remaining trigonometric functions of \( \theta \), we'll start from the given information: \( \cos \theta = \frac{4}{5} \) and \( 270^{\circ} < \theta < 360^{\circ} \), which places \( \theta \) in the fourth quadrant. Since \( \cos \theta = \frac{4}{5} \), we can identify \( a = 4 \) and \( r = 5 \) in the context of the right triangle definition of cosine, where the adjacent side is 4 and the hypotenuse is 5. We can find the opposite side \( b \) using the Pythagorean theorem: \[ a^2 + b^2 = r^2 \\ 4^2 + b^2 = 5^2 \\ 16 + b^2 = 25 \\ b^2 = 9 \\ b = 3 \] In the fourth quadrant, the opposite side will be negative, giving us \( b = -3 \). Now, we can find the sine and tangent functions: \[ \sin \theta = \frac{b}{r} = \frac{-3}{5} \\ \tan \theta = \frac{b}{a} = \frac{-3}{4} \] The other functions can be found using their relationships to sine and cosine: \[ \csc \theta = \frac{1}{\sin \theta} = \frac{5}{-3} = -\frac{5}{3} \\ \sec \theta = \frac{1}{\cos \theta} = \frac{5}{4} \\ \cot \theta = \frac{1}{\tan \theta} = \frac{4}{-3} = -\frac{4}{3} \] So, the trigonometric functions for \( \theta \) are: \[ \sin \theta = -\frac{3}{5}, \quad \cos \theta = \frac{4}{5}, \quad \tan \theta = -\frac{3}{4}, \quad \csc \theta = -\frac{5}{3}, \quad \sec \theta = \frac{5}{4}, \quad \cot \theta = -\frac{4}{3} \]

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