1. The problem statement, all variables and given/known data I was tutoring a student and could not answer one of his questions. Prove that y = mx +- square root( a^2*m^2 + b^2 ) is the equation for two lines passing through an ellipse 2. Relevant equations (x/a)^2 + (y/b)^2 = 1 is the equation of an ellipse 3. The attempt at a solution I started by suggesting he implicitly differentiate the equation for an ellipse and put it in the point slope equation, but it turned out to give an equation for dy/dx = m, but not the desired equation. I worked on different approaches at home for half an hour, but I'm stumped.