“ab＜0”是“曲线ax2+by2=1为双曲线”的（） A．充分不必要条件 B

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问题选择题

“ab＜0”是“曲线ax²+by²=1为双曲线”的（　　）

A．充分不必要条件

B．必要不充分条件

C．充分必要条件

D．既不充分又不必要条件

答案

一方面，由ab＜0，得a＞0，b＜0或a＜0，b＞0．

由此可知a与b符号相反，则方程表示双曲线，

反之另一方面，曲线ax²+by²=1化成

=1，

它表示双曲线时，必有ab＜0，故反之亦然．

故选C．

完形填空

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Now what are you waiting for?

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问答题

氧气是化工生产的重要原料，很多金属和非金属都能和氧气直接作用．

（1）第二周期某种元素形成的氧化物分子中，既含有σ键又含有π键且二者数目相同的物质的结构式为______，其中心原子杂化方式为______．氧元素与氟元素能形成OF₂分子，该分子的空间构型为______．氮元素的电负性______（填“＞”、“=”或“＜”）氧，主要原因是______．

（2）氧元素和过渡元素可形成多种价态的金属氧化物，如Cr₂O₃、Fe₂O₃、V₂O₅．基态铬原子外围电子排布式为______．核电荷数比Fe大1的元素在元素周期表中的位置为第______周期______族．

（3）四种同主族元素的氧化物晶体的晶格能数据如下表：

氧化物	MgO	CaO	SrO	BaO
晶格能/kJ•mol^-1	3791	3410	3223	3054

四种氧化物的共熔物冷却时，首先析出的是______．根据表中数据，说明影响上表中晶格能大小的主要因素______．

（4）某种由氧和钾元素形成的化合物，其晶胞结构如右图所示，该晶体的化学式为______．

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