Show that if any line through the variable point `A(k+1,2k)` meets the lines `7x+y-16=0,5x-y-8=0,x-5y+8=0` at `B ,C ,D ,` respectively, the `A C ,A B ,a n dA D` are in harmonic progression. (The three lines lie on the same side of point `Adot)dot`