a function is a relation between x and y, where an x can't go to two different y's
on a graph of f(x), no point can be directly above or below each other
this is known as the vertical line test: a vertical line cannot pass through two different points of the function
a circle is not a function
y=ax^2 +bx +c, a parabola the has a U shape, or upside down U shape is a function