I liked the organization and presentation of resource overall. <br /> However, there is an error defining vertical lines as &quot;functions.&quot; They could be described as a relation, but not a function. A function is defined as a relation of input and output values where each input corresponds to exactly one output. Vertical lines completely defy this, since for any input, say x=1 corresponds with an infinite number of outputs.<br /> Also, the plural form of axis is &quot;axes&quot;.