CP1: imported math for pi, used the area formula

A value is some object that exists, whereas a name is a reference to
that value. An example where it is important to distinguish between a
name and the value it represents is any reference to time. Sometimes
generalization is acceptable ("I don't have time") but often the actual
amount matters and so we care about the value, not the name itself.

Variables are useful in programming because their values are able to be
changed, by definition (variables can vary). This is useful because we
can use variables to parameterize our code, allowing for easier modification
and improvement or expansion. They can also make code more readable,
even without documentation because effective naming of variables allows
someone (even one who didn't write the specific code referenced) to see
the variable names and hopefully understand what the values mean and how
they are used in the program, for example, using "ballSpeed" vs "x".

circle_area.py

@@ -1,6 +1,9 @@
 # circle_area.py
 # --------------
 # By MWC Contributors
+import math
 
 print("This program will calculate the area of a circle.")
 radius = float(input("What is the circle's radius? "))
+area = math.pi * radius * radius
+print(area)

greetings.py

@@ -2,6 +2,6 @@
 # ------------
 # By MWC contributors
 
-my_name ="Chris"
+my_name = input('What is your name? ')
 greeting = "Hello, " + my_name
 print(greeting)

shapes.py

@@ -5,7 +5,19 @@
 from turtle import *
 
 def triangle(side_length):
-    pass
+    forward(side_length)
+    right(120)
+    forward(side_length)
+    right(120)
+    forward(side_length)
+    right(120)
 
 def rectangle(height, width):
-    pass
+    forward(width)
+    right(90)
+    forward(height)
+    right(90)
+    forward(width)
+    right(90)
+    forward(height)
+    right(90)