Question

19．若$\left\{\begin{array}{l}{x=-1}\\{y=2}\end{array}\right.$与$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$是方程mx+ny=0的两个解，则m+n=0．

Answer 1

分析 把x与y的两对值代入方程计算即可求出m+n的值．

解答 解：把$\left\{\begin{array}{l}{x=-1}\\{y=2}\end{array}\right.$与$\left\{\begin{array}{l}{x=2}\\{y=-1}\end{array}\right.$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，
故答案为：0

点评 此题考查了二元一次方程组的解，方程组的解即为能使方程组中两方程都成立的未知数的值．