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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §1.5 A Review of Trigonometry, #30 · Trigonometry · Identities

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Match $a sin x + b cos x$ with $A sin (x + ϕ)$ . From equation (9) show that $a = A cos ϕ$ and $b = A sin ϕ$ . Square and add to find $A =$ ____. Divide to find $tan ϕ = b / a$ .

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §1.5 A Review of Trigonometry, #30

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