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Solve: cos3theta+2costheta=0...

Solve: `cos3theta+2costheta=0`

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`cos3theta+2costheta=0`
`rArr 4cos^(3)theta-3costheta+2costheta=0`
`rArr 4cos^(3)theta-costheta=0`
`rArr costheta(4costheta^(2)-1)=0`
`rArr costheta=0` or `4cos^(2)theta-1=0`
Now `costheta=0`
`rArr theta=(2n+1)pi/2`.
and `4cos^(2)theta-1=0`
`rArr cos^(2)theta=1/4=(1/2)^(2)=cos^(2)pi/3`
`rArr theta=npi+-pi/3`.
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