对比y=sinx和y=-sinx的图像根据f（x）= sinwx在闭区间（-π/2,π/2）内是 减函数,初步估计f（x）= sinwx中得w＜0,那 sinwx的减区间为-π/2+2kπ≤wx≤π/2+2kπ即π/2w+2kπ/w≤x≤-π/2w+2kπ/w取k=0得π/2w≤x≤-...