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let x1=cos(pi/11)cos((2pi)/11)cos((3pi)/...

let `x_1=cos(pi/11)cos((2pi)/11)cos((3pi)/11)cos((4pi)/11)cos((5pi)/11)` and `x_2=cos(pi/11)+cos(2pi/11)+cos(3pi/11)+cos(4pi/11)+cos(5pi/11)` then show that `x_1.x_2=1/64(cosec(pi/22)-1)`

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