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Cos2 0-cos 20 = sin2 0

User SnakeDoc
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\begin{gathered} \cos ^2(x)-\cos (2x)=sen^2(x) \\ \text{ }\cos ^2(x)-sen^2(x)\text{ = }\cos (2x) \\ \end{gathered}

According to Double identities


\cos (2x)=2cos^2(x)-1
\begin{gathered} \cos ^2(x)-sen^2(x)=2\cos ^2(x)-1 \\ -sen^2(x)=2\cos ^2(x)-\cos ^2(x)-1 \\ -sen^2(x)=\cos ^2(x)-1 \\ 1=\cos ^2(x)+sen^2(x) \end{gathered}

User Dave Baghdanov
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