(t1-t2)/(t4-t0)=(t2-t0)/(t3-t4)
(t1-t2)*(t3-t4)=(t2-t0)* (t4-t0)
t1t3- t2t3- t1t4+ t2t4= t2t4- t0t4- t0t2+(t0)²
t1t3- t2t3+t0t2-(t0)² = t2t4- t0t4+t1t4- t2t4
t1t3- t2t3+t0t2-(t0)² = t4*(t2- t0+t1- t2)
t3*(t1- t2)+ t0*(t2- t0)
------------------------------- = t4
(t2- t0+t1- t2)