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#include <math.h>

double cos(double x);


This function computes the cosine of x (which should be given in radians).

Return Value

The cosine of x. If the absolute value of x is finite but greater than or equal to 2^63, the value is 1 (since for arguments that large each bit of the mantissa is more than Pi). If the value of x is infinite or NaN, the return value is NaN and errno is set to EDOM.


POSIX 1003.2-1992; 1003.1-2001


In general, this function's relative accuracy is about 1.7*10^(-16), which is close to the machine precision for a double. However, for arguments very close to Pi/2 and its odd multiples, the relative accuracy can be many times worse, due to loss of precision in the internal FPU computations. Since cos(Pi/2) is zero, the absolute accuracy is still very good; but if your program needs to preserve high relative accuracy for such arguments, link with `-lm' and use the version of cos from `libm.a' which does elaborate argument reduction, but is about three times slower.

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