You can put this solution on YOUR website! Circumference = 2πr
r = radius
1/2 diameter = r
so half 90 mm => 45 mm
Plug into 2πr => 2π(45)
Simplify by multiplying 45*2 => 90π
Thus your Circumference is 90π mm or ~282.74 mm
P.S. You noticed I first divided the diameter by two then later multiplied it by 2. This is unnecessary however it is how its written in most textbooks. For your future reference the Circumference of a circle is just diameter*pi
