could have been done ,could have
题目
could have been done ,could have
答案
来个都是虚拟.
could have done是主动,本来可以做而实际没做；
could have been done是被动,本来可以被做而实际没被做的.
举一反三
已知函数f（x）=x，g（x）=alnx，a∈R．若曲线y=f（x）与曲线y=g（x）相交，且在交点处有相同的切线，求a的值和该切线方程．
我想写一篇关于奥巴马的演讲的文章,写哪一篇好呢?为什么好
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