请把下列句子变为被动语态：
题目
请把下列句子变为被动语态：
我只知道被动语态的构成是：be＋动词过去分词
1.The teacher is keeping the pupils at school for a revision.
2.You must finish the article before Friday.
3.They promised Mary a new doll for her birthday.
答案
1.The pupils at school are kept for a revision by the teacher.2.The article must be finished by you before Friday.3.Mary was promised a new doll for her birthday by them.
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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