①PbO+C  $\frac{\underset{\_}{△}}{}$ Pb+CO （主要）

②PbO+CO    $\frac{\underset{\_}{△}}{}$ Pb+CO₂

③〔Cu （NH₃）₂〕Ac+CO+NH₃ $\frac{\underset{\_}{△}}{}$ 〔Cu （NH₃）₃〕Ac•CO；△H＜0（Ac^﹣代表醋酸根离子此反应无明显现象）

试根据图回答问题：

已知集合A={x∈R|ax²+2x+1=0，a∈R}中只有一个元素，求a的值并求出这个元素．

    Mr. Zhang and Mr. Wang work in the same office. They're good friends.Mr. Zhang says to Mr. Wang in the office,“I would like to have a small party in my home on Sunday evening. Would you and your wife like to come?”

    Mr. Wang says,“Thanks a lot. I'd like to. Let me tell my wife about it.”So Mr. Wang goes to the other office and calls his wife. About half an hour later, he comes back and looks very worried.

    “What's up?” asks Mr. Zhang,“Is your wife at home?”

    “No,” says Mr. Wang.“She isn't at home now , but my little daughter is in .My little daughter says her mother goes out to look for her.”