sin2π/5+cos26π/5+2cos3π*sin5π/6
=sin2π/5+cos(5π+π/5)+2cos(2π+π)*sin(π-π/6)
=sin2π/5-cosπ/5-2sinπ/6
=2sinπ/5cosπ/5-cosπ/5-1
=2×0.6×0.8-0.8-1
=-0.84