y=cos^2x-sinxcosx-3sin^2xsin^2x=1-cos^2x
y=4cos^2x-sinxcosx-3
y=2(2cos^2x)-1/2sin2x-3
y=2(cos2x+1)-1/2sin2x-3
y=2cos2x-1/2sin2x-1
y=根号(4+1/4)sin(2x+arctan(-4))-1asinc+bcosc=(a^2+b^2)^(1/2)*sin(c+arctan(b/a))
y=根号17/2*sin(2x-arctan4)-1
2x-arctan4=pai
x=pai/2+(arctan4)/2时,取最小值.(此时,x取值在(0,2pai)内
即:ymin=-(1+(根号17)/2)