It's a equation for finding all tangent lines to a circle when the point isn't on the circle itself. I think I remember that it was
But I can't remember if the x and y are in the correct place.
Edit- Let me show an example. Find the equation of all tangent lines that can be drawn from the point (5,0) to the circle (x-0)^2+(y-0)^2=9.
First I find the line equation.
Then I plug this into the equation, note I can only do this one because the center is at the origin because I still can't remember if the x or y are in the correct place but since they are both 0, it doesn't matter for me.
Uhhh then I blank out here... So I guess I need help solving it and remembering the equation correctly...