C acosh()

The acosh() function takes a single argument (x ≥ 1), and returns the arc hyperbolic cosine in radians. Mathematically, acosh(x) = cosh-1(x).

The acosh() function is included in <math.h> header file.

acosh() Prototype

double acosh(double x);

To find arc hyperbolic cosine of type int, float or long double, you can explicitly convert the type to double using cast operator.

 int x = 0;
 double result;
 result = acosh(double(x));

Also, two functions acoshf() and acoshl() were introduced in C99 to work specifically with type float and long double respectively.

float acoshf(float x);
long double acoshl(long double x);

acosh() Parameter and Return Value

The acosh() function takes a single argument greater than or equal to 1.

acosh() Return Value

The acosh() functions returns a number greater than or equal to 0 in radians. If the argument passed is less than 1 ( x < 1), the function returns NaN (not a number).

Example 1: acosh() function with different parameters

#include <stdio.h>
#include <math.h>

int main()
{
    // constant PI is defined
    const double PI =  3.1415926;
    double x, result;

    x =  5.9;
    result = acosh(x);
    printf("acosh(%.2f) = %.2lf in radians\n", x, result);

    // converting radians to degree
    result = acosh(x)*180/PI;
    printf("acosh(%.2f) = %.2lf in degrees\n", x, result);

    // parameter not in range
    x = 0.5;
    result = acosh(x);
    printf("acosh(%.2f) = %.2lf", x, result);

    return 0;
}

Output

acosh(5.90) = 2.46 in radians
acosh(5.90) = 141.00 in degrees
acosh(0.50) = nan

Example 2: acosh() for INFINITY and DBL_MAX

#include <stdio.h>
#include <math.h>
#include <float.h>

int main()
{
    double x, result;

    // maximum representable finite floating-point number
    x =  DBL_MAX;
    result = acosh(x);
    printf("Maximum value of acosh() in radians = %.3lf\n", result);

    // Infinity
    x = INFINITY;
    result = acosh(x);
    printf("When infinity is passed to acosh(), result = %.3lf\n", result);

    return 0;
}

Possible Output

Maximum value of acosh() in radians = 710.476
When infinity is passed to acosh(), result = inf

Here, DBL_MAX defined in float.h header file is the maximum representable finite floating-point number. And, INFINITY defined in math.h is a constant expression representing positive infinity.

Example 3: acoshf() and acoshl() function

#include <stdio.h>
#include <math.h>
int main()
{
    float fx, facosx;
    long double lx, ldacosx;

    // arc hyperbolic cosine of type float
    fx = 5.5054;
    facosx = acoshf(fx);

    // arc hyperbolic cosine of type long double
    lx = 5.50540593;
    ldacosx = acoshl(lx);

    printf("acoshf(x) = %f in radians\n", facosx);
    printf("acoshl(x) = %Lf in radians", ldacosx);

    return 0;
}

Output

acoshf(x) = 2.390524 in radians
acoshl(x) = 2.390525 in radians