||(s1x3+tx,s2x3+ty)-(s1x2+tx,s2x2+ty)||
/||(s1x2+tx,s2x2+ty)-(s1x1+tx,s2x1+ty)||
= ||(s1(x3-x2),s2(x3-x2))||/||(s1(x2-x1),s2(x2-x1))||
=sqrt(s1^2+s2^2)(x3-x2)/sqrt(s1^2+s2^2)(x2-x1)
=(x3-x2)/(x2-x1)