.
I was late for class yesterday __________ there was something wrong with my bike.
①BD平分∠CBF;
②FB2=FD•FA;
③AE•CE=BE•DE;
④AF•BD=AB•BF.
所有正确结论的序号是( )
则称函数f(x)是区间m,n上的“保值函数”,区间m,n称为“保值区间”.