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Ch. 3 - Trigonometric Identities and Equations
Blitzer - Trigonometry 3rd Edition
Blitzer3rd EditionTrigonometryISBN: 9780137316601Non è quello che usi tu?Cambia libro di testo
Capitolo 3, Problema 3

In Exercises 1–60, verify each identity. tan (-x) cos x = -sin x

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Start by recalling the trigonometric identity for the tangent of a negative angle: \( \tan(-x) = -\tan(x) \).
Substitute \( \tan(-x) \) with \( -\tan(x) \) in the given expression: \( \tan(-x) \cos(x) = -\tan(x) \cos(x) \).
Use the definition of tangent in terms of sine and cosine: \( \tan(x) = \frac{\sin(x)}{\cos(x)} \).
Substitute \( \tan(x) \) with \( \frac{\sin(x)}{\cos(x)} \) in the expression: \( -\tan(x) \cos(x) = -\left(\frac{\sin(x)}{\cos(x)}\right) \cos(x) \).
Simplify the expression by canceling \( \cos(x) \) in the numerator and denominator: \( -\sin(x) \). This shows that \( \tan(-x) \cos(x) = -\sin(x) \), verifying the identity.

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Trigonometric Identities

Trigonometric identities are equations that involve trigonometric functions and are true for all values of the variables involved. Common identities include the Pythagorean identities, reciprocal identities, and co-function identities. Understanding these identities is crucial for simplifying expressions and verifying equations in trigonometry.
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Tangent Function and Its Properties

The tangent function, defined as the ratio of the sine and cosine functions (tan(x) = sin(x)/cos(x)), has specific properties, including periodicity and symmetry. Notably, tan(-x) = -tan(x), which reflects the odd nature of the tangent function. This property is essential for manipulating and verifying trigonometric identities.
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Negative Angle Identities

Negative angle identities express the values of trigonometric functions for negative angles. For example, sin(-x) = -sin(x) and cos(-x) = cos(x). These identities help in transforming expressions involving negative angles into more manageable forms, which is particularly useful in verifying identities like the one presented in the question.
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