The cosine of d. If d is equal to double.NaN, double.NegativeInfinity, or double.PositiveInfinity, this method returns double.NaN.
The angle, d, must be in radians. Multiply by Math.PI/180 to convert degrees to radians.
Acceptable values of d range from approximately -9223372036854775295 to approximately 9223372036854775295. For values outside this range, the Math.Cos(double) method returns d unchanged rather than throwing an exception.