y=7-4sinxcosx+4cos^2x-4cos^4x=7-2sin2x+4cos^2x(1-cos^2x)=7-2sin2x+4cos^2xsin^2x=7-2sin2x+(2sinxcosx)^2=7-2sin2x+sin^2(2x)=6+1-2sin2x+sin^2(2x)=6+(sin2x+1)^2容易知道,当sin2x=-1时候,有最小值=6+0=6；当si...