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+---
+title: Math.acos()
+slug: Web/JavaScript/Reference/Global_Objects/Math/acos
+translation_of: Web/JavaScript/Reference/Global_Objects/Math/acos
+---
+<div>
+ {{JSRef("Global_Objects", "Math")}}</div>
+<h2 id="Summary" name="Summary">概述</h2>
+<p><code><strong>Math.acos()</strong></code> 返回一个数的反余弦值（单位为弧度），即：</p>
+<p><math display="block"><semantics><mrow><mo>∀</mo><mi>x</mi><mo>∊</mo><mo stretchy="false">[</mo><mrow><mo>-</mo><mn>1</mn></mrow><mo>;</mo><mn>1</mn><mo stretchy="false">]</mo><mo>,</mo><mspace width="thickmathspace"></mspace><mstyle mathvariant="monospace"><mrow><mo lspace="0em" rspace="thinmathspace">Math.acos</mo><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo></mrow></mstyle><mo>=</mo><mo lspace="0em" rspace="0em">arccos</mo><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo><mo>=</mo><mtext> the unique </mtext><mspace width="thickmathspace"></mspace><mi>y</mi><mo>∊</mo><mo stretchy="false">[</mo><mn>0</mn><mo>;</mo><mi>π</mi><mo stretchy="false">]</mo><mspace width="thinmathspace"></mspace><mtext>such that</mtext><mspace width="thickmathspace"></mspace><mo lspace="0em" rspace="0em">cos</mo><mo stretchy="false">(</mo><mi>y</mi><mo stretchy="false">)</mo><mo>=</mo><mi>x</mi></mrow><annotation encoding="TeX">\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</annotation></semantics></math></p>
+<h2 id="Syntax" name="Syntax">语法</h2>
+<pre class="syntaxbox">Math.acos(<em>x</em>) </pre>
+<h2 id="Parameters" name="Parameters">参数</h2>
+<dl>
+ <dt>
+  <code>x</code></dt>
+ <dd>
+  一个数值</dd>
+</dl>
+<h2 id="Description" name="Description">描述</h2>
+<p><code>acos</code> 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 <code>NaN</code>。</p>
+<p>由于 <code>acos</code> 是 <code>Math</code> 的静态方法，所以应该像这样使用：<code>Math.acos()</code>，而不是作为你创建的 <code>Math</code> 实例的属性（<code>Math</code> 不是一个构造函数）。</p>
+<h2 id="Examples" name="Examples">示例</h2>
+<h3 id="Example:_Using_Math.acos" name="Example:_Using_Math.acos">例子：使用 <code>Math.acos</code></h3>
+<pre class="brush:js">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
+</pre>
+<p>对于小于 -1 或大于 1 的值，<code>Math.acos</code> 返回 <code>NaN</code>。</p>
+<h2 id="规范">规范</h2>
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">规范版本</th>
+   <th scope="col">规范状态</th>
+   <th scope="col">注解</th>
+  </tr>
+  <tr>
+   <td>ECMAScript 1st Edition. Implemented in JavaScript 1.0</td>
+   <td>Standard</td>
+   <td>Initial definition.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES5.1', '#sec-15.8.2.2', 'Math.acos')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-math.acos', 'Math.acos')}}</td>
+   <td>{{Spec2('ES6')}}</td>
+   <td> </td>
+  </tr>
+ </tbody>
+</table>
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+<p>{{ CompatibilityTable() }}</p>
+<div id="compat-desktop">
+ <table class="compat-table">
+  <tbody>
+   <tr>
+    <th>Feature</th>
+    <th>Chrome</th>
+    <th>Firefox (Gecko)</th>
+    <th>Internet Explorer</th>
+    <th>Opera</th>
+    <th>Safari</th>
+   </tr>
+   <tr>
+    <td>Basic support</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+   </tr>
+  </tbody>
+ </table>
+</div>
+<div id="compat-mobile">
+ <table class="compat-table">
+  <tbody>
+   <tr>
+    <th>Feature</th>
+    <th>Android</th>
+    <th>Chrome for Android</th>
+    <th>Firefox Mobile (Gecko)</th>
+    <th>IE Mobile</th>
+    <th>Opera Mobile</th>
+    <th>Safari Mobile</th>
+   </tr>
+   <tr>
+    <td>Basic support</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+    <td>{{ CompatVersionUnknown() }}</td>
+   </tr>
+  </tbody>
+ </table>
+</div>
+<h2 id="See_also" name="See_also">相关链接</h2>
+<ul>
+ <li>The {{jsxref("Global_Objects/Math", "Math")}} object it belongs to.</li>
+ <li>{{jsxref("Math.asin()")}}</li>
+ <li>{{jsxref("Math.atan()")}}</li>
+ <li>{{jsxref("Math.atan2()")}}</li>
+ <li>{{jsxref("Math.cos()")}}</li>
+ <li>{{jsxref("Math.sin()")}}</li>
+ <li>{{jsxref("Math.tan()")}}</li>
+</ul>