Solution
= Solution
A <vector> from the point on $L_2$ to the point on $L_1$ is
$$
\boxed{\mathbf R(\lambda,\mu)=\mathbf a-\mathbf c+\lambda\mathbf b-\mu\mathbf d.}
$$
Choosing the opposite direction negates this <vector> and does not change its <norm> or the shortest distance.