The formula for circumference is 
.
To express the circumference exactly, plug the diameter in for
and put the 
symbol in front of it. Don't multiply. This expresses the exact circumference.
Say you have the diameter of 17.
You can express the circumference with 17.
To express the circumference exactly, plug the diameter in for
Say you have the diameter of 17.
You can express the circumference with 17