M+N=K
N=K-M代入:
MX+NY+1=0
MX+(K-N)Y+1=0
M(X-Y)+KY+1=0
令:KY+1=0,X-Y=0
得:X=Y=-1/K
所以:MX+NY+1=0恒过点(-1/K,-1/K)