From the notebook
Using the Diameter Directly in Circumference Questions
If a circle has diameter 14 cm, its circumference can be found directly:
C = πd = 14π cm
As a decimal, that is about 43.98 cm.
Relation to 2πr
Because the diameter is twice the radius, πd and 2πr are the same formula written in different ways.
Quick choice: if the question gives the diameter, use πd. If it gives the radius, use 2πr.
Useful next step: cover the worked answer and try the same method on a similar question before moving on.