则当 时, .
①f(x1+x2)=f(x1)•f(x2)②f(x1•x2)=f(x1)+f(x2)
③ f(x1)−f(x2)x1−x2<0 ④ f(x1+x22)<f(x1)+f(x2)2
当 f(x)=(12)x 时,上述结论中正确的序号是{#blank#}1{#/blank#}.