View source for Relationship between secant, Gudermannian, and cosh

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<strong>Theorem:</strong> The following formula holds:
$$\sec(\mathrm{gd}(x))=\cosh(x),$$
where $\sec$ denotes the [[secant]], $\mathrm{gd}$ denotes the [[Gudermannian]], and $\cosh$ denotes the [[cosh|hyperbolic cosine]]. 
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<strong>Proof:</strong> █ 
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