原式=sin[π-(5π/6-4x)]
=sin(4x+π/6)
=-sin(-4x-π/6)
=-cos[π/2-(-4x-π/6)]
=-cos(4x+2π/3)
=-cos[2(2x+π/3)]
=-[1-2sin²(2x+π/3)]
=-7/9