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问题 选择题

把少量下列物质分别放在水中，充分搅拌，不能得到溶液的是（　　）

A．植物油

B．碳酸钠

C．氢氧化钠

D．蔗糖

答案

植物油不溶于水，与水混合后形成的是乳浊液，不均一，碳酸钠、氢氧化钠、蔗糖都易溶于水，能形成均一稳定的混合物．

故选A．

解答题
（1）求右焦点坐标是（2，0），且经过点( -2 ，-
2
 )的椭圆的标准方程；
（2）求与椭圆
x2
24
+
y2
49
=1有共同的焦点并且与双曲线
x2
36
-
y2
64
=1有共同渐近线的双曲线方程．
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填空题

Americans’ (1) impulses keep generating surprises. Charitable giving plays an even larger role in the (2) . Demand for nonprofit services gets proportionately bigger as a locality’s (3) rises. The philanthropy of the wealthy may not hinge on tax (4) to the degree many believe. The US (5) the world in levels of charitable activity. Some experts see charity as a (6) trait of the US, more than (7) or business. But those forces may be (8) , as many nonprofits, from healthcare to classical music are selling (9) in a marketplace alongside for-profit (10) . Charity is no mere (11) activity. It pays off for society in ways that may (12) the rates of return on many traditional investments. Charity not only helps those on the (13) end but also strengthens the (14) of society at large. Moreover, it appears to make the givers themselves more (15) . The pattern that conservatives are better givers than (16) is less about politics than about charity-linked (17) most common to conservatives, religious commitment, marriage and children, and entrepreneurship. The main point is that more Americans, regardless of ideology, embrace giving as a tool for (18) . The urge to make a difference, and to take (19) in it, outweighs (20) considerations.
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