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Ch. 1 - Trigonometric Functions
Lial - Trigonometry 12th Edition
Lial12th EditionTrigonometryISBN: 9780136552161, 9780135440780, 9780136881117, 9780135440827, 9780135924891Non è quello che usi tu?Cambia libro di testo
Capitolo 2, Problema 79

Find the indicated function value. If it is undefined, say so. See Example 4. sec 1800°

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1
Recall that the secant function is the reciprocal of the cosine function, so \(\sec \theta = \frac{1}{\cos \theta}\).
Since the angle given is \(1800^\circ\), reduce it to an equivalent angle between \(0^\circ\) and \(360^\circ\) by subtracting multiples of \(360^\circ\): calculate \(1800^\circ - 5 \times 360^\circ\).
Simplify the expression to find the equivalent angle: \(1800^\circ - 1800^\circ = 0^\circ\).
Evaluate \(\cos 0^\circ\), knowing that \(\cos 0^\circ = 1\).
Find \(\sec 1800^\circ\) by taking the reciprocal of \(\cos 0^\circ\), so \(\sec 1800^\circ = \frac{1}{1}\).

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Understanding the Secant Function

The secant function, sec(θ), is the reciprocal of the cosine function, defined as sec(θ) = 1/cos(θ). It is undefined wherever cos(θ) equals zero, which occurs at odd multiples of 90°. Knowing this helps determine when sec(θ) is defined or undefined.
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Graphs of Secant and Cosecant Functions

Angle Coterminality and Reduction

Angles differing by full rotations (360°) share the same trigonometric values. To simplify large angles like 1800°, subtract multiples of 360° to find a coterminal angle between 0° and 360°, making it easier to evaluate the function.
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Coterminal Angles

Evaluating Trigonometric Functions at Standard Angles

Trigonometric functions have known values at standard angles such as 0°, 90°, 180°, and 270°. Recognizing these values allows quick evaluation of functions like sec(θ) once the angle is reduced, facilitating identification of defined or undefined values.
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Drawing Angles in Standard Position