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if cos 2theta=1/3 and 0 less than or equal to 2theta less than or equal to pi, find cos theta
7 years ago

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Steve
since 0 <= 2θ <= π,
0 <= θ <= π/2
That is, θ is in QI.

cos2θ = 2cos^2θ-1
so cosθ = √((1+cos2θ)/2) = √((1+1/3)/2) = √(2/3)
In the first quadrant, we don't have to worry about ± signs.
7 years ago

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