Exponential functions can be expressed in the form [math]f\left(x\right)=y_0e^{kt}[/math] where [math]y_0[/math] and [math]k[/math] are real numbers, called parameters. Parameter [math]y_0[/math] controls the vertical scaling of the function and parameter [math]k[/math] affects the growth rate of the function. Function [math]f\left(x\right)=y_0e^{kx}[/math] is a model for [b]exponential growth[/b] if [math]k>0[/math] and a model for [b]exponential decay[/b] if [math]k<0[/math].[br][br]Two observations:[br][list][*]The graph of [math]f[/math] crosses the [math]y[/math]-axis at [math]y_0[/math].[/*][*]The slope of [math]f[/math] as it crosses the [math]y[/math]-axis is the product [math]y_0k[/math].[/*][/list]