cos

# File ext/bigdecimal/lib/bigdecimal/math.rb, line 85
  def cos(x, prec)
    raise ArgumentError, "Zero or negative precision for cos" if prec <= 0
    return BigDecimal("NaN") if x.infinite? || x.nan?
    n    = prec + BigDecimal.double_fig
    one  = BigDecimal("1")
    two  = BigDecimal("2")
    x = -x if x < 0
    if x > (twopi = two * BigMath.PI(prec))
      if x > 30
        x = twopi
      else
        x -= twopi while x > twopi
      end
    end
    x1 = one
    x2 = x.mult(x,n)
    sign = 1
    y = one
    d = y
    i = BigDecimal("0")
    z = one
    while d.nonzero? && ((m = n - (y.exponent - d.exponent).abs) > 0)
      m = BigDecimal.double_fig if m < BigDecimal.double_fig
      sign = -sign
      x1  = x2.mult(x1,n)
      i  += two
      z  *= (i-one) * i
      d   = sign * x1.div(z,m)
      y  += d
    end
    y
  end