Start with the hyperbolic paraboloid surface z = f(x, y) = y^2 − x^2, and an arbitrary point P(a, b, c) on the surface.
A) What is c in terms of a and b?
B) Find the scalar equation of the tangent plane at P, and write it as a function z = T(x, y).
C) Equating f(x, y) = T(x, y) gives a quadratic expression for (x, y) values where the hyperbolic paraboloid surface intersects the tangent plane. Completing the squares in this expression leads to a difference of squares that nicely factors into the form P1(x, y)P2(x, y) = 0.
D) Choose one of these factors to be zero, say P1(x, y) = 0. It is the equation of a plane. The line of intersection of this plane with the tangent plane z = T(x, y) gives one of these magical lines — construct the parametric equations for this line. (Hints: see example #7 in Section 12.5, and point P is on the line.)
E) Verifying that every point on this line lies on the surface z = y^2 − x^2 checks your math.