Find the area of a circle with a diameter of 4cm.

The diameter of a circle is the longest measurement across the circle, and is a straight line passing through the middle. Half of this line is the radius, which is a straight line from the centre of the circle to the edge. If the diamater is 4, the radius is therefore 2, as half of 4 is 2. 

To find the area of a circle, we use the formula pir². This is the value of pi multiplied by the value of r squared. 2² is 22, which = 4. We then multiply this by pi to get 4pi, or 12.6 (to 1 d.p.)