答案是-cosA
sin(2π-α)=-sinasin(π/2+α)=cosacos(-5π/2+α)=cos(5π/2-a)=cos(2π+π/2-a)=cos(π/2-a)=sinasin(3π-α)=sin(2π+π-a)=sin(π-a)=sinacos(3π/2+α)=cos(π+π/2+a)=-cos(π/2+a)=-[-sina]=sina原式=-sinacosasina/(sinasina)=-cosa