# How do I find the directrix of a hyperbola?

The directrix is the vertical line $x = \frac{{a}^{2}}{c}$.
For a hyperbola ${\left(x - h\right)}^{2} / {a}^{2} - {\left(y - k\right)}^{2} / {b}^{2} = 1$,
where ${a}^{2} + {b}^{2} = {c}^{2}$,
the directrix is the line $x = {a}^{2} / c$.