--- Math.acos(x)
--- Math.asin(x)
--- Math.atan(x)

    x εջѴؿͤ饸֤ޤ

    ֤ͤϰϤϤ줾 [0, +][-/2, +/2]
    (-/2, +/2) Ǥ

    acos(x), asin(x) Ǥ x  -1.0 <= x <= 1 ϰǤ
    Фʤޤ(̡NaN ֤ޤ)

    ruby 1.7 feature: acos(), asin() ϰϳΰФơ
     Errno::EDOM [Errno::EXXX] ȯޤ

