Вычислите: cos(6pi/5) * sin(3pi/10) - sin(6pi/5) * sin (-4pi/5)

cos(π+π/5)sin(π/2-π/5)-sin(π+π/5)*(-sin(π-π/5))==-cosπ/5*cosπ/5-(-sinπ/5)*(-sinπ/5)=-cos²π/5-sin²π/5=-(cos²π/5+sin²π/5)=-1