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      <text>4.3.3.9 Summary</text>
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      <text><![CDATA[<h4>Summary</h4>
<p>We have covered a large number of trigonometric identities. The most important of them and probably the ones most worth memorising are given in the following Key Point.</p>

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<h4>Key Point<br></h4>
\[\begin{aligned}
\cos^2\theta+\sin^2\theta&amp;\equiv 1\\
\sin 2\theta&amp;\equiv 2\sin\theta\cos\theta\\
\cos 2\theta&amp;\equiv \cos^2\theta-\sin^2\theta\\
&amp;\equiv 2\cos^2\theta -1\\
&amp;\equiv 1-2\sin^2\theta\\
\sin (A\pm B)&amp;\equiv \sin A\cos B\pm\cos A\sin B\\
\cos (A\pm B)&amp;\equiv \cos A\cos B\mp\sin A\sin B
\end{aligned}\]
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