How to find areas of circles using the formula: A =pr2.
The area of a circle of radius 'r' is given by:
NOTE: The value of p can never be known exactly, so areas of circles cannot be calculated exactly. Common approximations for p are: 3.14, and 22/7.
Example 1 if 'r' = 5 for a given circle, and p = 3.14, then the area of the circle is:
Also, if the area of the circle is known, then the radius can be calculated like this:
Example 2 if the area of a circle is 125, and p = 3.14, then the radius of the circle is found as follows:
Maths Helper Plus can show the
calculations and also draw a diagram for calculating the area of a circle given the
radius, or for calculating the radius given the area.
Step 2 Display the parameters box
Press the F5 key to display the parameters box:
'Area of Circle.mhp' solves two kinds of problems:
To solve a problem like example 1 above, where you calculate the area of a circle given the radius:
To solve a problem like example 2 above, where you calculate the radius of a circle given the area:
Click on the 'B' edit box and type the value of p that you are using. often this will be 3.14, but may also be 22/7. For greatest accuracy, type 'pi' for p.
Click the 'Update' button to refresh the diagram and calculations.
Step 3 Adjust the scale of the labelled diagram
If the labelled diagram does not display the entire circle, the scale needs to be reduced. In this case, briefly press the F10 key enough times until the whole circle is seen.
You can make the diagram bigger by holding down 'Ctrl' while you press F10.