sinx+siny=1/3,求cosx+cosy的取值范围

（sinx+siny）^2+(cosx+cosy)^2
=2+2(sinxsiny+cosxcosy)
=2+2cos(x-y)
2cos(x-y)值域[-2,2]
（sinx+siny）^2=1/9
(cosx+cosy)^2属于[-1/9,35/9]且(cosx+cosy)^2>=0得解