tan(π/4-x)= - tan(x-π/4)设x-π/4=t（这是方便理解）y=tant在定义域上递增所以 y= - tant在定义域上递减.y递减时,-π/2 +kπ＜t＜π/2 +kπ-π/2 +kπ＜x-π/4＜π/2 +kπ-π/4+kπ＜x＜3π/4 +kπ递减区间（-π/...