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
*x*– or*y*-axis.- If the given coordinates of the focus have the form [latex]\left(p,0\right)[/latex], then the axis of symmetry is the
*x*-axis. Use the standard form [latex]{y}^{2}=4px[/latex]. - If the given coordinates of the focus have the form [latex]\left(0,p\right)[/latex], then the axis of symmetry is the
*y*-axis. Use the standard form [latex]{x}^{2}=4py[/latex].

- If the given coordinates of the focus have the form [latex]\left(p,0\right)[/latex], then the axis of symmetry is the
- Multiply [latex]4p[/latex].
- Substitute the value from Step 2 into the equation determined in Step 1.

### Example 4: Writing the Equation of a Parabola in Standard Form Given its Focus and Directrix

What is the equation for the **parabola** with **focus** [latex]\left(-\frac{1}{2},0\right)[/latex] and **directrix** [latex]x=\frac{1}{2}?[/latex]

### Solution

The focus has the form [latex]\left(p,0\right)[/latex], so the equation will have the form [latex]{y}^{2}=4px[/latex].

- Multiplying [latex]4p[/latex], we have [latex]4p=4\left(-\frac{1}{2}\right)=-2[/latex].
- Substituting for [latex]4p[/latex], we have [latex]{y}^{2}=4px=-2x[/latex].

Therefore, the equation for the parabola is [latex]{y}^{2}=-2x[/latex].

### Try It 4

What is the equation for the parabola with focus [latex]\left(0,\frac{7}{2}\right)[/latex] and directrix [latex]y=-\frac{7}{2}?[/latex]