Respuesta :
[tex]\bf \textit{Pythagorean Identities}
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sin^2(\theta)+cos^2(\theta)=1\implies cos^2(\theta)=1-sin^2(\theta)
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cos(\theta)=\pm\sqrt{1-sin^2(\theta)}\\\\
-----------------------------\\\\
thus
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cos(x)=\pm \sqrt{1-(0.9)^2}[/tex]