都乘以sinx
原式=sinx*cosx*cos2x*cos4x*cos8x/sinx
=1/2sin2x*cos2x*cos4x*cos8x/sinx
=1/4sin4x*cos4x*cos8x/sinx
=1/8sin8x*cos8x/sinx
=1/16 sin16x/sinx
后面就很简单了吧.