$x=-\sqrt{25-y^{2}}$
$x=\sqrt{25-y^{2}}$

$y=-\sqrt{25-x^{2}}$
$y=\sqrt{25-x^{2}}$

$x=\sqrt{25-y^{2}}$
$x=-\sqrt{25-y^{2}}\text{, }|y|\leq 5$

$y=\sqrt{25-x^{2}}$
$y=-\sqrt{25-x^{2}}\text{, }|x|\leq 5$