求cosπ/5乘cos2π/5乘cos3π/5乘cos4π/5
题目
求cosπ/5乘cos2π/5乘cos3π/5乘cos4π/5
答案
乘以sinπ/5再除以sinπ/5
原式 = sinπ/5乘cosπ/5乘cos2π/5乘cos3π/5乘cos4π/5除sinπ/5
= 0.5乘sin2π/5乘cos2π/5乘cos3π/5乘cos4π/5除sinπ/5
= 0.25乘sin4π/5乘cos3π/5乘cos4π/5除sinπ/5
= 0.125乘sin8π/5乘cos3π/5除sinπ/5
= -0.125乘sin3π/5乘cos3π/5除sinπ/5
=-(1/16)乘sin6π/5除sinπ/5
= 1/16
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