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Given that cos(theta) = -2 radical 5/5, and is in Quadrant II, what is cos(2theta)?

Given that cos(theta) = -2 radical 5/5, and is in Quadrant II, what is cos(2theta)?
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Given that cos(theta) = -2 radical 5/5, and is in Quadrant II, what is cos(2theta)?

User Tom Benyon
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cos \theta = ( - 2 √(5) )/(5)


cos 2\theta = 2cos {}^(2) \theta - 1 \\ cos2 \theta = 2( ( - 2 √(5) )/(5) ) {}^(2) - 1 \\ cos2 \theta = 2( (20)/( \\ 25) ) - 1 \\ cos2 \theta = (40 - 25)/(25) = (15)/(25) = (3)/(5)

User Milot
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