--- Math.acosh(x) ruby 1.7 feature
--- Math.asinh(x) ruby 1.7 feature
--- Math.atanh(x) ruby 1.7 feature

    x εжؿ֤ͤޤ

        asinh(x) = log(x + sqrt(x * x + 1))
        acosh(x) = log(x + sqrt(x * x - 1)) [x >= 1]
        atanh(x) = log((1+x)/(1-x)) / 2     [-1 < x < 1]

