Question

7．方程組$\left\{\begin{array}{l}2x+y=a\\ x+y=3\end{array}\right.$的解為$\left\{\begin{array}{l}x=2\\ y=b\end{array}\right.$，則a、b的值分別為（　　）

• A． 1，2
• B． 5，1
• C． 2，1
• D． 2，3

Answer 1

分析 把$\left\{\begin{array}{l}x=2\\ y=b\end{array}\right.$代入方程組$\left\{\begin{array}{l}2x+y=a\\ x+y=3\end{array}\right.$，即可解答．

解答 解：把$\left\{\begin{array}{l}x=2\\ y=b\end{array}\right.$代入方程組$\left\{\begin{array}{l}2x+y=a\\ x+y=3\end{array}\right.$得：$\left\{\begin{array}{l}{4+b=a}\\{2+b=3}\end{array}\right.$
解得：$\left\{\begin{array}{l}{a=5}\\{b=1}\end{array}\right.$
故選：B．

點評 本題主要考查了二元一次方程組的解，解題的關鍵是用代入法進行求解．