Show that the graph of $y=a x^4+b x^2+c$ has the property that if it contains the point $\left(x_1, y_1\right)$, then it also contains the point $\left(-x_1, y_1\right)$. Any function whose graph has this property is called an even function. Notice that the graph of an even function is symmetric with respect to the $y$-axis.