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Prove that the roots of the equation 8...

Prove that the roots of the equation
`8x^(3)-4x^(2)-4x+1=0 " are " cos""pi/7, cos""(3pi)/7 " and " cos""(5pi)/7`.
Evaluate `sec""pi/7+sec""(3pi)/7+sec""(5pi)/7`

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The correct Answer is:
Required equation is , `z^(3)-21z^(2)+35z-7=0` whose roots are
`tan^(2). (pi)/(7), tan^(2). (3pi)/(7), tan^(2). (5pi)/(7)`
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