The range of the cosine function is -1 and 1 inclusive. The numeric expression is treated as an angle in the current trigonometric mode: RADians or DEGrees. The default trigonometric mode is RADians.
COMPLEX Arguments
COS accepts either a COMPLEX or REAL argument and returns a value of the same type. For COMPLEX arguments the angle must be specified in radians, regardless of the current trigonometric mode. The real and imaginary parts of COS(Z) are calculated (using real arithmetic) as
REAL(COS(Z)) = COS(REAL(Z))*COSH(IMAG(Z))
IMAG(COS(Z)) = -SIN(REAL(Z))*SINH(IMAG(Z))
Notice that intermediate values generated during the calculation of the function can cause over or underflow errors for very large or small values of Z.