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