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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §1.5 A Review of Trigonometry, #16 · Trigonometry · Identities

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Show that $(cos t + i sin t)^{2} = cos 2 t + i sin 2 t$ , if $i^{2} = - 1$ .

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §1.5 A Review of Trigonometry, #16

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