Line passing through points A(x1,y2),B(x2,y2) is given by
y−y1=(y2−y1x2−x1)(x−x1)
Equation of line joining (at1,at1) and (at2,at2) is
y−at1=(at2−at1)at2−at1(x−at1)t1y−at1=(a(t1−t2)t1t2)a(t2−t1)(x−at1)t1y−at1=−1t1t2(x−at1)(t1y−a)t2=−(x−at1)t1t2y−at2=−x+at1t1t2y+x=a(t1+t2)