原函数y=3tan（π/4-2x）=3tan（2x-π/4）∵y=tanx的单调增区间为（-π/2+kπ,π/2+kπ）,k∈Z无单调减区间∴令-π/2+kπ＜2x-π/4＜π/2+kπ,解得-π/8+kπ/2＜x＜3π/8+kπ/2∴函数y=3tan(π/4-2x)的单调减区间为（-π/8+kπ/2,3π/8+kπ/2）,k∈Z（PS：我不希望提问的得不到答案,所以挑靠后的零回答；采纳时回答速度选很快,回答态度选很认真,）