Some functions are even, some are odd, and some are neither.
An even function is a function whose graph is symmetric across the y-axis. Plot the graph of the cosine function on both sides of the y-axis. If you fold the graph on the y-axis, notice that the graph on the right of the axis will lie directly on top of the graph on the left of the axis. An odd function is a function whose graph is symmetric about the lorigin. Plot the graph of the sine function on both sides of the y-axis, then place your finger on the origin and rotate the graph 180 degerees.