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<a name="acosh_002c-acoshf_002d_002d_002dinverse-hyperbolic-cosine"></a>
<h3 class="section">1.4 <code>acosh</code>, <code>acoshf</code>&mdash;inverse hyperbolic cosine</h3>
<a name="index-acosh"></a>
<a name="index-acoshf"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;math.h&gt;
double acosh(double <var>x</var>);
float acoshf(float <var>x</var>);

</pre></div>
<p><strong>Description</strong><br>
<code>acosh</code> calculates the inverse hyperbolic cosine of <var>x</var>.
<code>acosh</code> is defined as 
</p><div class="smallexample">
<pre class="smallexample"> log(<var>x</var> + sqrt(<var>x</var>*<var>x</var>-1))
</pre></div>

<p><var>x</var> must be a number greater than or equal to 1.
</p>
<p><code>acoshf</code> is identical, other than taking and returning floats.
</p>
<br>
<p><strong>Returns</strong><br>
<code>acosh</code> and <code>acoshf</code> return the calculated value.  If <var>x</var> 
less than 1, the return value is NaN and <code>errno</code> is set to <code>EDOM</code>.
</p>
<br>
<p><strong>Portability</strong><br>
Neither <code>acosh</code> nor <code>acoshf</code> are ANSI C.  They are not recommended
for portable programs.
</p>
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