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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §1.5 A Review of Trigonometry, #15 · Trigonometry · Identities

By averaging … and … in (4–5) find a formula for

By averaging $cos (s - t)$ and $cos (s + t)$ in (4–5) find a formula for $cos s cos t$ . Find a similar formula for $sin s sin t$ .

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §1.5 A Review of Trigonometry, #15

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