pkg load optim;

function y = f (x)

  y = [
    (
      (x(3) - x(1)) * (x(7) - x(5))  +
      (x(4) - x(2)) * (x(8) - x(6))  
    ) ^ 2,
     (
      (x(11) - x(9)) * (x(7) - x(5))  +
      (x(12) - x(10)) * (x(8) - x(6))  
    ) ^ 2];
  y(3) = 0;y(4) = 0;y(5) = 0;y(6) = 0;y(7) = 0;y(8) = 0;
  y(9) = 0;y(10) = 0;y(11) = 0;y(12) = 0;
endfunction

x0 = [100, 100, 600, 600, 700, 600, 900, 100, 1100, 100, 1600, 600];
#x = fminunc(@f, reshape(x0, 12,1));
x = fsolve(@f, x0); #WORKS!

#x = x0;
l1 = [x(1), x(2); x(3), x(4)];
l2 = [x(5), x(6); x(7), x(8)];
l3 = [x(9), x(10); x(11), x(12)];

plot(l1(:,1), l1(:,2), l2(:,1), l2(:,2), l3(:,1), l3(:,2),'-');