下面方程组中.解为$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$的是( )A．$\left\{\begin{array}{l}{x-y=3}\\{3x-y=1}\end{array}\right.$B．$\left\{\begin{array}{l}{x-y=-1}\\{3x+y=-5}\end{array}\right.$C� 题目和参考答案——青夏教育精英家教网—

题目内容

9．下面方程组中，解为$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$的是（　　）

A．

$\left\{\begin{array}{l}{x-y=3}\\{3x-y=1}\end{array}\right.$

B．

$\left\{\begin{array}{l}{x-y=-1}\\{3x+y=-5}\end{array}\right.$

C．

$\left\{\begin{array}{l}{x-2y=3}\\{3x+5y=-5}\end{array}\right.$

D．

$\left\{\begin{array}{l}{x-y=-1}\\{3x+y=5}\end{array}\right.$

分析根据二元一次方程组的解的定义，把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$分别代入方程组的两个方程中，使两个方程同时成立的方程组即为所求．

解答解：A、把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程x-y=3，左边≠右边，所以$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$不是原方程组的解，故本选项不符合题意；
B、把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程x-y=-1，左边=右边；把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程3x+y=-5，左边≠右边；所以$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$不是原方程组的解，故本选项不符合题意；
C、把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程x-2y=3，左边≠右边，所以$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$不是原方程组的解，故本选项不符合题意；
D、把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程x-y=-1，左边=右边；把$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$代入方程3x+y=5，左边=右边；所以$\left\{\begin{array}{l}{x=1}\\{y=2}\end{array}\right.$是原方程组的解，故本选项符合题意；
故选D．

点评本题考查了二元一次方程组的解的定义：一般地，二元一次方程组的两个方程的公共解，叫做二元一次方程组的解．