A circle has an area of 81TT mm^2.
Work out the diameter of the circle.
The diameter of the circle is 9mm.
To calculate the diameter, use the formula A = πr^2, where A is the area of the circle, and r is the radius.
Therefore, r = √(81TT/π) = 3√(81TT).
The diameter is twice the radius, so the diameter is 2 × 3√(81TT) = 9mm.
I suspect Anthony means A = 81π mm^2
so
π R^2 = 81 π
R = 9 mm
D = 18 mm