The hyperbolic cosine function is defined by cosh(x) = (e^x + e^-x)/2. Prove that cosh is an even function.

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A function f(x) is even if f(x) = f(-x)

`cosh x = (e^x + e^(-x))/2`

`cosh(-x) = (e^(-x) + e^-(-x))/2`

=> `(e^-x + e^x)/2`

As can be seen f(x) = f(-x).

This proves that cosh(x) is an even function.

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