Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §6.7 Hyperbolic Functions, #58 · Calculus · Differentiation rules

Derive … from equation (5)

Derive $cos i x = cosh x$ from equation (5). What is the cosine of the imaginary angle $i = - 1$ ?

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §6.7 Hyperbolic Functions, #58

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