Question

How do you convert `r = -9 cos theta` into rectangular form?

Answer 1

`x^2+9x+y^2=0`

Explanation:

Since `r=sqrt(x^2+y^2)`, if you multiply both sides by `r` you have

`r^2 = -9rcos(theta)`

Now, `r^2 = x^2+y^2`, and `rcos(theta)=x`.

So, the equation becomes

`x^2+y^2 = -9x`, which is a circle.