Complete greeting and circle_area scripts.

- A value can have many different types but a name is just a reference to a value.
- When you are filling out a form you are providing values for names. If the form
asks you to do something like "combine lines 1 and 3" your answer better not be "line 1 line 3".
- When you need to repeat a value, like the area of a circle, many times it is easier
and more efficient to save the value to a variable. It also makes code more understandable.

circle_area.py

@@ -2,5 +2,9 @@
 # --------------
 # By MWC Contributors
 
+from math import pi
+
 print("This program will calculate the area of a circle.")
 radius = float(input("What is the circle's radius? "))
+area = pi * radius * radius
+print(area)