解方程组：y=kx-2,y^2=8x（kx-2)^2=8x,整理为：k^2x^2-(4k+8)x+4=0则:x1+x2=(4k+8)/k^2,x1x2=4/k^2则:|x1-x2|=√[(x1+x2)^2-4x1x2]=√[(4k+8)^2/k^4-4*4/k^2]=8√(k+1)/k^2线段AB的中点的横坐标是2,则|x1-x2|/2＝8√...