若x1,x2∈R,x1≠x2,则下列性质对函数f(x)=2x成立的是 _________ .(把满足条件的序号全部写在横线上)①f(x1+x2)=f(x1)·f(x2)②f(x1·x2)=f(x1)+f(x2)③[f(x1)﹣f(x2)]·(x1﹣x2)>0④
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