# Java Math acos()

#### The Java Math acos() method returns the arc cosine of the specified value.

The arc cosine is the inverse of the cosine function.

The syntax of the `acos()` method is:

``Math.acos(double num)``

Here, `acos()` is a static method. Hence, we are accessing the method using the class name, `Math`.

## acos() Parameters

The `acos()` method takes a single parameter.

• num - number whose arc cosine is to be returned. It should be always less than 1.

## acos() Return Value

• returns the arc cosine of the specified number
• returns `NaN` if the specified number is `NaN` or greater than 1

Note: The returned value is an angle between 0.0 to pi.

## Example 1: Java Math acos()

``````import java.lang.Math;

class Main {
public static void main(String[] args) {

// create variable
double a = 0.5;
double b = 0.79;
double c = 0.0;

// print the arc cosine value
System.out.println(Math.acos(a));  // 1.0471975511965979
System.out.println(Math.acos(b));  // 0.6599873293874984
System.out.println(Math.acos(c));  // 1.5707963267948966
}
}``````

In the above example, we have imported the `java.lang.Math` package. This is important if we want to use methods of the `Math` class. Notice the expression,

``Math.acos(a)``

Here, we have directly used the class name to call the method. It is because `acos()` is a static method.

## Example 2: Math acos() Returns NaN

``````import java.lang.Math;

class Main {
public static void main(String[] args) {

// create variable
double a = 2;

// square root of negative number
// results in not a number (NaN)
double NaN = Math.sqrt(-5);

// print the arc cosine value
System.out.println(Math.acos(a));  // NaN
System.out.println(Math.acos(NaN));  // NaN
}
}``````

Here, we have created two variables named a and b.

• Math.acos(a) - returns NaN because value of a is greater than 1.
• Math.acos(b) - returns NaN because square root of a negative number (-5) is not a number.

Note: We have used the Java Math.sqrt() method to compute the square root of a number.