I keep getting “Focus and directrix” questions wrong in Conic Sections. What's the trap?
confusing the focus (a point) with the directrix (a line).
The usual trap: confusing the focus (a point) with the directrix (a line). How to avoid it: focus is a point inside; directrix is a line outside. Go back to the definition — a conic is the set of points whose distances to a focus and a directrix are in the fixed ratio e.
Key point
Focus and directrix
Common mistake
confusing the focus (a point) with the directrix (a line).
Memory tip
focus is a point inside; directrix is a line outside.
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