
sin6°sin42°sin66°sin78°
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原式=sin6cos48cos24cos12
=2sin6cos6cos12cos24cos48/2cos6
=sin12cos12cos24cos48/2cos6
=2sin12cos12cos24cos48/4cos6
=sin24cos24cos48/4cos6
=sin48cos48/8cos6
=sin96/16cos6
=sin(90+6)/16cos6
=cos6/16cos6
=1/16
=2sin6cos6cos12cos24cos48/2cos6
=sin12cos12cos24cos48/2cos6
=2sin12cos12cos24cos48/4cos6
=sin24cos24cos48/4cos6
=sin48cos48/8cos6
=sin96/16cos6
=sin(90+6)/16cos6
=cos6/16cos6
=1/16
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