prove that: cosπ/7*cos2π/7*cos4π/7=-1/8
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Formulae used: sin 2A = 2sinAcosA
Multiplied with 2/2 in step-2 and 3
2sinAcosB=sin(a+b) + sin(a-b)
Multiplied with 2/2 in step-2 and 3
2sinAcosB=sin(a+b) + sin(a-b)
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see the attachment for the solution.
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