LEARN · EXPLAIN · REVISE

Find the x-coordinate of the directrix.

Read the idea, work independently, then explain what changed.

高二選擇性必修第一册(A版).pdf · 3.3 · PDF 135 / printed page 130

Revisit first: Hyperbola

TOPIC 01

Parabola

Relate focus, directrix and vertex, and retain conditions in line-parabola intersections.

Relate focus, directrix and vertex, and retain conditions in line-parabola intersections.
Justify the method and check the conditions in a new situation.

Use one hint at a time. A correction explains what changed, not just the final answer.

01 / Foundation#Your turn

y^2=24x

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Translate the geometric condition into a coordinate equation.

Hint 2

Use this intermediate relation.

\text{directrix}:x=-a

Worked solution

The requested value is -6.

Checks and common pitfalls: Focus and directrix are on opposite sides of the vertex.

Think first. Reveal a hint when the class is ready.

Teacher preparation and assessment

Relate focus, directrix and vertex, and retain conditions in line-parabola intersections.
Which condition is essential in parabola?
How does changing the sign of a alter y²=4ax?

Focus-directrix: y²=4ax has focus (a,0) and directrix x=−a.
Nondegeneracy: a must be nonzero; a parabola has a vertex and no center of symmetry.