# How do you use the vertical line test?

$y = 3 x + 4$ is a line and passes the vertical line test, it is a function.
$y = {x}^{2}$ is a parabola and passes the vertical line test, it is a function.
${x}^{2} + {y}^{2} = 1$ is a circle and fails the vertical line test, it is not a function.