# Math.acos()

## 概述

Math.acos() 返回一个数的反余弦值（单位为弧度），即：

$∀ x ∊ [ - 1 ; 1 ] , Math.acos ( x ) = arccos ( x ) = the unique y ∊ [ 0 ; π ] such that cos ( y ) = x \forall x \in [{-1};1],\;\mathtt{\operatorname{Math.acos}(x)} = \arccos(x) = \text{ the unique } \; y \in [0; \pi] \, \text{such that} \; \cos(y) = x$

## 语法

Math.acos(x)

## 参数

x

## 描述

acos 方法以 -1 到 1 的一个数为参数，返回一个 0 到 pi （弧度）的数值。如果传入的参数值超出了限定的范围，将返回 NaN

## 示例

### 例子：使用 Math.acos

Math.acos(-2);  // NaN
Math.acos(-1);  // 3.141592653589793
Math.acos(0);   // 1.5707963267948966
Math.acos(0.5); // 1.0471975511965979
Math.acos(1);   // 0
Math.acos(2);   // NaN


## 规范

ECMAScript 1st Edition. Implemented in JavaScript 1.0 Standard Initial definition.
ECMAScript 5.1 (ECMA-262)
Math.acos
Standard
ECMAScript 2015 (6th Edition, ECMA-262)
Math.acos
Standard

## 浏览器兼容性

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