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trig identity for cos(3Ф)

Answer 1

The trigonometric triple-angle identities give a relationship between the basic trigonometric functions applied to three times an angle in terms of trigonometric functions of the angle itself. Triple-angle Identities.
sin3θ=3sinθ−4sin3θ
cos3θ=4cos3θ−3cosθ

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Answer 2

The trigonometric triple-angle identities give a relationship between the basic trigonometric functions applied to three times an angle in terms of trigonometric functions of the angle itself. Triple-angle Identities.
sin3θ=3sinθ−4sin3θ
cos3θ=4cos3θ−3cosθ