y=4－2cosx＋2sinx－sinxcosx ===>> 设sinx－cosx=t,则：t²=1－2sinxcosx,sinxcosx=(1－t²)/2此时,y=4＋2t－(1/2)＋(1/2)t²=(1/2)[t＋2]²＋(3/2) ======>>>> t∈[－√2,√2] y的最大值是当t=√2...