They tell you that conic sections are exactly what the words describe : Slice a double cone, the edge of the slice is a conic section, parabola, hyperbola, ellipse.
Then they tell you that y = x^2 is a parabola, or that all second degree equations in x and y are conic sections, or worst of all, they come up with the focus/directrix definition.
NOBODY shows you how to get the equation from the sliced cone !!!!!!!!!!!!
Well, here goes – (the math is after the picture, and it is so simple)
