I was studying the parametric form of equation of a line and I am facing problems with some signs.
$$\frac{x-x_1}{\sin\theta}=\frac{y-y_1}{\cos\theta}=r$$
Now I am always having problem regarding the sign of r whether it's $+r$ or $-r$ whenever I don't know the distance between any two point.
For example I want to find the equation of a line such that its segment between the line $5x-y+4=0$ and $3x+4y-4=0$ is bisected at $(1,5)$
Now if I write two equation using the point $(1,5)$ with which line should I use $+r$ and $-r$ is unclear and hence I don't know what to do in such situation. Can you please suggest me a way to easily figure out how I should chose my $r$.