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1.5 asin, asinf—arc sine

Synopsis 

     #include <math.h>
     double asin(double x);
     float asinf(float x);

Description

asin computes the inverse sine (arc sine) of the argument x. Arguments to asin must be in the range −1 to 1.

asinf is identical to asin, other than taking and returning floats.

You can modify error handling for these routines using matherr.
Returns
asin returns values in radians, in the range of -pi/2 to pi/2.

If x is not in the range −1 to 1, asin and asinf return NaN (not a number), set the global variable errno to EDOM, and issue a DOMAIN error message.

You can change this error treatment using matherr.