问题
选择题
a，b为正实数是a2+b2≥2ab的（ ）
A．充分不必要条件
B．必要不充分条件
C．充要条件
D．既不充分也不必要条件
答案
∵当a，b为正实数时一定可以看出重要不等式成立，
当a2+b2≥2ab成立时，不一定推出a，b都是正数，这里对数字的正负没有限制，
故前者可以推出后者，而后者不能推出前者，
∴前者是后者的充分不必要条件，
故选A．
搜题请搜索小程序“爱下问搜题”
a，b为正实数是a2+b2≥2ab的（ ）
A．充分不必要条件
B．必要不充分条件
C．充要条件
D．既不充分也不必要条件
∵当a，b为正实数时一定可以看出重要不等式成立，
当a2+b2≥2ab成立时，不一定推出a，b都是正数，这里对数字的正负没有限制，
故前者可以推出后者，而后者不能推出前者，
∴前者是后者的充分不必要条件，
故选A．
| 完形填空。 | ||||
| People in many countries are learning English. Some learn at school, others study by 1 . A few learn English 2 the radio. Why do all these people want to learn English? It's difficult 3 that question. Many boys and girls learn English at school 4 it is one of their subjects. Some people learn English because it's useful for their work. Many students often learn English for their 5 studies because 6 the college, some of their books are 7 in English. It is not easy to learn a foreign language. But there is 8 difficult in the world if you 9 your heart into 10 .
|