阅读下面材料，在空白处填上适当的内容（一个单词）或括号内单词的正确形式。

    We high school students do have some growing pains, but we can get rid of them____ (correct) and wisely. First, some of us are upset about our body style and looks. It's unnecessary and it's not important at all. We needn't care about it. It is one's inner beauty that____ (matter). Second, we sometimes seem to be misunderstood by our teachers, parents and classmates. Faced with this, we can find a proper time to have a heart-to-heart talk with them, trying to remove the misunderstanding. Some of us have fewer friends, I think being open-minded and friendly will do us good. Third, we may fall behind others, ____ makes us stressed. Actually we can encourage____ to work efficiently, full of____ (determine). At last, some of us don't have much pocket money, so they feel unhappy. Isn't it strange? So long as we have some, that's enough. And we can learn how to spend money.

如图，点 $O$ 是矩形 $ABCD$ 的对角线 $AC$ 的中点， $M$ 是 $CD$ 边的中点．若 $AB=8$ ， $OM=3$ ，则线段 $OB$ 的长为____．