sinx+siny=2/3,cosx+cosy的取值范围
法一:t=cosx+cosy
t^2+(sinx+siny)^2=t^2+(2/3)^2=(cosx+cosy)^2+(sinx+siny)^2=2+2cos(x-y)≤4
t∈[-4√2/3,4√2/3]
当x=y+2kπ时取得极值.
法二:拉格朗日乘数法