Question

13．已知$\left\{\begin{array}{l}{x=3}\\{y=2}\end{array}\right.$是二元一次方程組$\left\{\begin{array}{l}{mx+3y=2}\\{nx+3y=8}\end{array}\right.$的解，則m-n的值是（　�。�

• A． 1
• B． 2
• C． -2
• D． -1

Answer 1

分析 把x與y的值代入方程組計(jì)算求出m與n的值，代入原式計(jì)算即可得到結(jié)果．

解答 解：把$\left\{\begin{array}{l}{x=3}\\{y=2}\end{array}\right.$代入方程組得：$\left\{\begin{array}{l}{3m+6=2}\\{3n+6=8}\end{array}\right.$，
解得：$\left\{\begin{array}{l}{m=-\frac{4}{3}}\\{n=\frac{2}{3}}\end{array}\right.$，
則m-n=-2，
故選C

點(diǎn)評 此題考查了二元一次方程組的解，方程組的解即為能使方程組中兩方程都成立的未知數(shù)的值．