--- 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]

    acosh(x) Ǥ x x >= 1 ϰǤʤФʤޤ
    (̡㳰 Errno::EDOM [Errno::EXXX] ȯޤ)

    atanh(x) Ǥ x  -1.0 < x < 1 ϰǤʤФ
    ޤ(̡㳰 Errno::EDOM [Errno::EXXX] ȯޤ)

