题目

Sometimes you’ll hear people say that you can’t love others until you love yourself. Sometimes you’ll hear people say that you can’t expect someone else to love you until you love yourself. Either way, you’ve got to love yourself first and this can be tricky. Sure we all know that we’re the apple of our parents’ eyes, and that our Grandmas think we’re great talents and our Uncle Roberts think that we will go to the Olympics, but sometimes it’s a lot harder to think such nice thoughts about ourselves. If you find that believing in yourself is a challenge, it is time you（1999•温州）如图，已知△ABC的周长为30cm，D，E，F分别为AB，BC，CA的中点，则△DEF的周长等于    cm．