An equation like sin2 u + cos2 u = 1 is called an identity because it holds for all values of u. An equation like sin u = 0.5 is not an identity because it holds only for selected values of u, not all. If you differentiate both sides of a trigonometric identity in u with respect to u, the resulting new equation will also be an identity. Differentiate the following to show that the resulting equa-tions hold for all u. a. sin 2u = 2 sin u cos u b. cos 2u = cos2 u - sin2 u

Trigonometry (MindTap Course List)
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Author:Charles P. McKeague, Mark D. Turner
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Chapter5: Identities And Formulas
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An equation like sin2 u + cos2 u = 1 is called an identity because it holds for all values of u. An equation like sin u = 0.5 is not an identity because it holds only for selected values of u, not all. If you differentiate both sides of a trigonometric identity in u with respect to u, the resulting new equation will also be an identity. Differentiate the following to show that the resulting equa-tions hold for all u.

a. sin 2u = 2 sin u cos u

b. cos 2u = cos2 u - sin2 u

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