Using a protractor.
Diameter: 2*4 = 8 cm
The area of a circle (in square hectometers) if the diameter is 4 cm is: 0.001257 hectometers2
Its diameter is about 4 cm rounded
2 cm
Radius = 1/2 of diameter = 4 cm
12.57 square cm.
Its radius is: 4/2 = 2 cm
The circumference = Pai x Diameter = 3.14 x 4 = 12.56 cm
diameter =2*radius =2*2 cm = 4 cm
This indicates a radius of 4 cm, in which case the area of the circle is: 50.27 square cm
10 cm
diameter = 12.568/pi = 4.00051865 or about 4 cm.