若A(m+1,n-1,3).B(2m,n,m-2n).C(m+3,n-3,9)三点共线，则m+n_____
热心网友
AB (3m+1,2n-1,m-2n+3)BC (3m+3,2n-3,m-2n+9)AB=kBC3m+1=k(3m+3).......(1)2n-1=k(2n-3).......(2)m-2n+3=k(m-2n+9)...(3)(1)/(2): (3m+1)(2n-3)=(3m+3)(2n-1)2n+3m=0............(4)(4)代入(2)/(3)(-3m-1)/(m+3m+3)=(-3m-3)/(m+3m+9)m=0 n=0m+n=0A(1,-1,3) B(0,0,0) C(3,-3,9)