Question

20．已知關(guān)于x，y的二元一次方程組$\left\{\begin{array}{l}{2ax+by=3}\\{ax-by=1}\end{array}\right.$的解為$\left\{\begin{array}{l}{x=1}\\{y=-1}\end{array}\right.$，則a-2b的值是（　�。�

• A． -2
• B． 2
• C． 3
• D． -3

Answer 1

分析 把$\left\{\begin{array}{l}{x=1}\\{y=-1}\end{array}\right.$代入方程組，得出關(guān)于a、b的方程組，求出方程組的解即可．

解答 解：把$\left\{\begin{array}{l}{x=1}\\{y=-1}\end{array}\right.$代入方程組$\left\{\begin{array}{l}{2ax+by=3}\\{ax-by=1}\end{array}\right.$得：$\left\{\begin{array}{l}{2a-b=3}\\{a+b=1}\end{array}\right.$，
解得：$\left\{\begin{array}{l}{a=\frac{4}{3}}\\{b=-\frac{1}{3}}\end{array}\right.$，
所以a-2b=$\frac{4}{3}$-2×（-$\frac{1}{3}$）=2，
故選B．

點(diǎn)評(píng) 本題考查了解二元一次方程組和二元一次方程組的解，能得出關(guān)于a、b的方程組是解此題的關(guān)鍵．