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Find the value of 2cos^3(pi/7)-cos^2(pi/...

Find the value of `2cos^3(pi/7)-cos^2(pi/7)-cos(pi/7)`

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`theta=pi/7`
`=2cos^3theta-cos^2theta-costheta`
`=costheta(2cos^2theta-1)-cos^2theta`
`=costheta(cos2theta)-cos^2theta`
`=costheta(cos2theta-costheta)`
`=cos(-2sin3/2thetasintheta/2)`
`=-2cos(pi/7)sin(pi/14)sin(3/14pi)`
`=-2cos(pi/7)cos(pi/2-pi/14)`
...
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