<xref>

<theorem xml:id="thm-Pythagorus">
    <title>(Pythagorean Theorem)</title>
    <statement>
    <p>
        In a right triangle with side lengths <m>a,b</m> and hypotenuse having
        length <m>c</m>, we have <m>c^2 = a^2 + b^2.</m>
    </p>
    </statement>
</theorem>
 
<theorem xml:id="thm-law-of-cosines">
    <title>(Law of Cosines)</title>
    <statement>
    <p>
        In a triangle with two sides having lengths <m>a,b</m> and included
        angle <m>\theta</m>, the length, <m>c</m>, of the third side is given by
        <me>
        c^2 = a^2 + b^2 - 2ab\cos\theta.
        </me>
    </p>
    </statement>
</theorem>
 
<p>We see that <xref ref="thm-Pythagorus" text="type-global-title" />, is a
special case of <xref ref="thm-law-of-cosines" /> by setting <m>\theta=\pi/2.</m>
</p>
 
<p>We see that the
<xref ref="thm-Pythagorus" text="custom">Pythagorean Theorem</xref>
is a special case of the
<xref ref="thm-law-of-cosines" text="custom">Law of Cosines</xref>
by setting <m>\theta=\pi/2</m>.
</p>
 
<p>By the
<xref ref="thm-law-of-cosines" text="type-phrase-global">Law of Cosines, Theorem</xref>,
we know that if <m>a=b</m>, and <m>\theta=\pi/3</m>,
the triangle must be equilateral.
</p>
 
<p>
Theorems like <xref ref="thm-Pythagorus,thm-law-of-cosines"></xref>
belong in a section on
<xref provisional="trigonometry"/> that I shall have to write.
</p>