Question

19．若$\left\{\begin{array}{l}{x=-1}\\{y=2}\end{array}\right.$與$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$是方程mx+ny=0的兩個解，則m+n=0．

Answer 1

分析 把x與y的兩對值代入方程計算即可求出m+n的值．

解答 解：把$\left\{\begin{array}{l}{x=-1}\\{y=2}\end{array}\right.$與$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$代入方程mx+ny=0得：$\left\{\begin{array}{l}{-m+2n=0①}\\{2m-n=0②}\end{array}\right.$，
①+②得：m+n=0，
故答案為：0

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(shù)的值．