f（x）=cos(x/2) -sin(x/2) 单调区间,值域

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sin(α±β)=sin αcos β±cos αsin β
cos(x/2) -sin(x/2)=根号2*sin(45º-x/2)
单调区间-π/2+2nπ＜45º-x/2＜π/2+2nπ 恒单增,2nπ＜x＜π+2nπ
恒单减,π+2nπ＜2π+2nπ
值域为（-根号2,根号2）