y=sinXcosX-2(sinX+cosX）+4,令t=sinX+cosX,t2=1+2sinXcosX,sinXcosX=1/2(t2-1),y(t)=1/2t2-1/2-2t+4=1/2t2-2t+3/4,对称轴t=2,代入y(t)得-5/4