--- Math.cosh(x) ruby 1.7 feature
--- Math.sinh(x) ruby 1.7 feature
--- Math.tanh(x) ruby 1.7 feature

    x жؿ֤ͤޤ

        cosh(x) = (exp(x) + exp(-x)) / 2
        sinh(x) = (exp(x) - exp(-x)) / 2
        tanh(x) = sinh(x) / cosh(x)

