已知O为△ABC的外心，且 $\overrightarrow{OA}+\overrightarrow{OB}=\overrightarrow{CO}$ ，则△ABC的形状是____．

期中考试越来越近了，我的心理压力越来越大，经常失眠，还会因为一点儿小事发脾气，我快崩溃了。”假如你是这位同学的好朋友，你会（    ）

_____his timely help , Kate overcame the hardship at last.                                                                                                                                                                                                                                                                                                                                                                                                                         

$\sqrt{2}$ 的倒数是（   ）

已知a^m=2，aⁿ=3，则a^4m﹣3n的值是（   ）