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Exercise 10.2 · Q2

Q.Find the coordinates of the focus, axis of the parabola, the equation of the directrix and the length of the latus rectum of the parabola x2=6yx^2 = 6y.

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✓ Free question

The parabola x2=6yx^2 = 6y opens upward with vertex at the origin; comparing with x2=4ayx^2 = 4ay gives a=32a = \frac{3}{2}, so the focus is (0,32)\left(0, \frac{3}{2}\right), directrix y=−32y = -\frac{3}{2}, axis the yy-axis, and latus rectum 66.

Why this form tells us everything

A parabola is the locus of points equidistant from a fixed point (the focus) and a fixed line (the directrix). When the equation is written as x2=4ayx^2 = 4ay, the parabola has its vertex at the origin and opens along the yy-axis. The parameter aa encodes the "focal distance"—how far the focus sits from the vertex—and from it we can read off every geometric feature.

The standard form x2=4ayx^2 = 4ay describes a parabola that:

  • has vertex at (0,0)(0, 0),
  • opens upward if a>0a > 0 (downward if a<0a < 0),
  • has focus at (0,a)(0, a),
  • has directrix y=−ay = -a,
  • has the yy-axis as its axis of symmetry,
  • has latus rectum (the chord through the focus perpendicular to the axis) of length 4a4a.

Our task is to identify aa and then extract these features.


Step-by-step extraction

1. Identify the parameter aa

We have x2=6yx^2 = 6y. Comparing with the standard form x2=4ayx^2 = 4ay, we see

4a=6⇒a=64=32.4a = 6 \quad \Rightarrow \quad a = \frac{6}{4} = \frac{3}{2}.

Since a>0a > 0, the parabola opens upward.

2. Find the focus

The focus of x2=4ayx^2 = 4ay lies at (0,a)(0, a). Substituting a=32a = \frac{3}{2},

Focus=(0,32).\text{Focus} = \left(0, \frac{3}{2}\right).

3. Write the equation of the directrix

The directrix is the horizontal line y=−ay = -a. With a=32a = \frac{3}{2},

Directrix: y=−32.\text{Directrix: } y = -\frac{3}{2}.

4. State the axis of the parabola

The axis of symmetry is the line passing through the vertex and focus, perpendicular to the directrix. Here it is the yy-axis, or

Axis: x=0.\text{Axis: } x = 0.

5. Calculate the length of the latus rectum

The latus rectum is the chord through the focus perpendicular to the axis. Its length is always 4a4a. Thus

Length of latus rectum=4a=4⋅32=6.\text{Length of latus rectum} = 4a = 4 \cdot \frac{3}{2} = 6.

Tip

For any parabola x2=4ayx^2 = 4ay, the latus rectum length is simply the coefficient of yy—here 66—so you can read it directly without computing 4a4a separately.


Summary table

FeatureValue
Focus(0,32)\left(0, \frac{3}{2}\right)
Directrixy=−32y = -\frac{3}{2}
Axisx=0x = 0 (the yy-axis)
Latus rectum length66
✓Final answer

The focus is (0,32)\left(0, \frac{3}{2}\right), the axis is the line x=0x = 0, the directrix is y=−32y = -\frac{3}{2}, and the length of the latus rectum is 66.

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