Added the text "The domain of y=e^x..." (#20980)

guide/english/mathematics/e-constant/index.md

@@ -12,6 +12,8 @@ E is a constant which we can use to observe the behavior of exponential function
 Furthermore, the inverse of e^x is ln(x), the base logarithmic function. 
 OF particular importance, ln(e) = 1.
 
+The domain of y=e^x is (-infinity,infinity).
+
 http://www.mathsisfun.com/numbers/e-eulers-number.html
 #### More Information:
 More information about e, exponential, and logarithmic functions can be found [here](https://www.khanacademy.org/tag/natural-logarithm-and-e)