Area of a Circle
Consider the first figure and suppose that one semicircle is cut from the center near the circumference in each direction. Then the semicircle is spread out as in the second figure. The length is the length of half of the circumference of the circle.
Suppose that the other half of the circle is cut in the same way and fitted into the first, as shown by the dashes in the second figure. It is evident that if we make a large number of cuts, the figure formed will approximate a rectangle whose length is equal to onehalf of the circumference and whose width is equal to the radius. We note that the rectangle is or and this rectangle has the same area as the circle.
Rule: The area of a circle equals times the square of the radius, or the area of a circle is equal to onefourth times the square of the diameter.
If area, circumference, diameter, radius, then

(if radius is given)

(if diameter is given)

(if circumference and radius are given)
Example:
A reinforced concrete beam is to have a total area of steel of square cm. How many bars must be used if they are to be cm in diameter?
Solution:
Given that the total area of reinforced concrete beam is square cm  (1)
Area of the circle of cm. diameter
square cm  (2)
Number of bars (approx)
Example:
A paper is in the form of a rectangle , where cm and cm. A semicircular portion with as the diameter is cut off. Find the area of the remaining paper.
Solution:
Area of rectangle square cm
Area of semicircle
square cm
Required area of the remaining paper square cm.