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      <text>9.5.1.3 Key point</text>
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      <text><![CDATA[<p>The final result in the previous Example is true in general:</p>
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<h4>Key Point</h4>
<p>If <span class="math inline">\(\ell\)</span> and <span class="math inline">\(m\)</span> are the direction cosines of a vector lying in the <span class="math inline">\(xy\)</span> plane, then <span class="math inline">\(\ell^2+m^2=1.\)</span></p>
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