Parabolas Centered at the Origin
Writing Equations of Parabolas in Standard Form
In the previous examples, we used the standard form equation of a parabola to calculate the locations of its key features. We can also use the calculations in reverse to write an equation for a parabola when given its key features.
HOW TO
Given its focus and directrix, write the equation for a parabola in standard form.
- Determine whether the axis of symmetry is the - or -axis.
- Multiply .
- Substitute the value from Step 2 into the equation determined in Step 1.
EXAMPLE 3
Writing the Equation of a Parabola in Standard Form Given its Focus and Directrix
What is the equation for the parabola with focus and directrix ?
Solution
The focus has the form , so the equation will have the form .
Therefore, the equation for the parabola is .
TRY IT #3
What is the equation for the parabola with focus and directrix ?