Determine the length of the circumference for a circle of radius 21mm. Construction: 1. With the center O draw a circle of radius 21mm. Draw a diameter AB. 2. Draw a tangent AC equals 3 times of AB. 3. Draw a radius OD making an angle of 30º with OB. 4. From D draws a line DE perpendicular to line OB. Background music is taken from No. 3 by The Denotes | Music promoted by Creative Commons Attribution-ShareAlike 3.0 Unported #Circumference #Engineeringdrawing #Technicaldrawing