热心网友
sinx +siny=1/3,==-2/3≤siny≤1,u=sinx-cosy^2=1/3-siny-1+siny^2=(siny-1/2)^2-11/12,最大值=(-2/3-1/2)^2-11/12=4/9,最小值=-11/12.
热心网友
由已知sinx+siny=1/3，== sinx=(1/3)-siny,则：u=sinx-(cosy)^2=(1/3)-siny-[1-(siny)^2]=(siny)^2-siny-(2/3)=(siny)^2-siny+(1/4)-(1/4)-(2/3)=[siny-(1/2)]^2-(11/12)当siny=-1时，u(max)=4/3当siny=1/2时，u(min)=-11/12