Question 128490

Notice if we graph {{{f(x)=x^2-4}}}, we get 



{{{ graph( 500, 500, -10, 10, -10, 10, x^2-4) }}} 




From the graph, we can see that the curve violates the horizontal line test. In other words, if you pass a straightedge horizontally through the graph, the straightedge will touch two points. So the function is not one-to-one.