How to write a circle

Here's two ways:
- set of points
  $S^1 = \set{(x,y) \st x^2 + y^2 = 1}$
- parametric
  $\gamma(t) = (\cos t, \sin t)$

Good for different purposes:
- set of points good for checking points
  (like $(3/5, 4/5)$)
  and generalizes to higher dimensions easily
- parametric is good for -drawing- it
  [and admits some generalizations, namely spherical coordinates,
   but it's more complicated.]

Finding tangent lines/planes to both these forms is basic
multivariable calculus.
