First, you need to know what a diameter and radius are.
A diameter is 2 times the size of a radius and it stretches from one end of the circle to the other side going through the middle. The diameter must go through the middle. If not, it is a chord.
The radius is half the size of diameter. That is, if a diameter of a circle is 10 centimeters, the radius is 5 centimeters. If the radius of a circle is 3, its diameter is 6.
Look below to see a picture:
Second, you need to know the formulas for finding the circumference and area of a circle. To find the area of a circle, you use the following formula:
or 
To find the circumference of a circle, you need to use the following formula:
or C = 
x 2r.Third, you need to know that
= 3.14 or 22/7. When finding the radius and diameter given the area and circumference of a circle, please use the following three steps.
Step 1: Write down all the information you know and put a question mark next to the information you need to find.
For example, if you are given a question that asks you to find the radius of a circle with the circumference of 628 cms and they tell you to use 3.14 for
. There are certain pieces of information that you have. Make a box of what you now.c = 628 cms
= 3.14r = ?
Step 2: Write down the formula of either area or circumference depending on what informatin you were given in the question.
In our case, we are given information on the circumference. Thus, we need the formula for the circumference.
or C = x 2r.Step 3: Substitute all the values you know into the equation and solve for the unknown.
In this case, it is the radius.
We know the circumference, the value for
, and we need to find the radius.628 = 2(3.14) r
628 = 6.28r
Do inverse operation of multiplication to cancel out 6.28 so that r is left alone. Your answer should be: 100
