题目内容
12.证明:$\left\{\begin{array}{l}{x+y>2}\\{xy-(x+y)+1>0}\end{array}\right.$是$\left\{\begin{array}{l}{x>1}\\{y>1}\end{array}\right.$的充要条件.分析 证明:根据充分条件和必要条件的定义进行证明即可.
解答 即xy-(x+y)+1=(x-1)(y-1),
若$\left\{\begin{array}{l}{x+y>2}\\{xy-(x+y)+1>0}\end{array}\right.$成立,则xy-(x+y)+1=(x-1)(y-1)>0,
即$\left\{\begin{array}{l}{x>1}\\{y>1}\end{array}\right.$或$\left\{\begin{array}{l}{x<1}\\{y<1}\end{array}\right.$,
∵x+y>2,∴$\left\{\begin{array}{l}{x<1}\\{y<1}\end{array}\right.$,(不成立),$\left\{\begin{array}{l}{x>1}\\{y>1}\end{array}\right.$成立.
若$\left\{\begin{array}{l}{x>1}\\{y>1}\end{array}\right.$,则$\left\{\begin{array}{l}{x+y>2}\\{xy-(x+y)+1>0}\end{array}\right.$成立,
即$\left\{\begin{array}{l}{x+y>2}\\{xy-(x+y)+1>0}\end{array}\right.$是$\left\{\begin{array}{l}{x>1}\\{y>1}\end{array}\right.$的充要条件.
点评 本题主要考查充分条件和必要条件的证明,根据充分条件和必要条件的定义是解决本题的关键.
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