# How do you convert r = 2a cosθ into a cartesian equation?

${x}^{2} + {y}^{2} = 2 a x$
Multiply both sides by $r$ to get ${r}^{2} = 2 a r \cos \theta$. Use the fact that ${r}^{2} = {x}^{2} + {y}^{2}$ and that $x = r \cos \theta$ to get ${x}^{2} + {y}^{2} = 2 a x$.