Question

5．已知$\left\{\begin{array}{l}{x=2}\\{y=1}\end{array}\right.$是關于x，y的二元一次方程組$\left\{\begin{array}{l}{x-5y=a}\\{3x+5y=b}\end{array}\right.$的解，則a+b的值是（　�。�

• A． 1
• B． 3
• C． 6
• D． 8

Answer 1

分析 把x與y的值代入方程組計算求出a與b的值，即可確定出原式的值．

解答 解：把$\left\{\begin{array}{l}{x=2}\\{y=1}\end{array}\right.$代入方程組得：$\left\{\begin{array}{l}{2-5=a}\\{6+5=b}\end{array}\right.$，即$\left\{\begin{array}{l}{a=-3}\\{b=11}\end{array}\right.$，
則a+b=-3+11=8，
故選D

點評 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的值．