Circle measuring lesson

Measuring Circles lesson

Using formulas to find all measurements of a circle

Using Radius and Diameter

When you know one value you can easily find the other.

So when you know the radius, you can find the diameter or when you know the diameter, you can find the radius of any circle.

Formula for Radius and Diameter;

r = 1/2d

d = 2r

Try it out:

Radius measures 8, what would the diameter be?

When radius = 8

 $r\ =\ 8$ 

  $8\ =\ \frac{1}{2}d$  
1.To find the diameter, do the inverse operation by multiplying both sides by 2
2. You now have :  $\left(2\right)8\ =\ \left(2\right)\ \frac{1}{2}d$  

3. Solve thru the equation

 $\left(2\right)8\ =\ \left(2\right)\ \frac{1}{2}d$  

  $16=\ \frac{2}{2}d$  


  $16\ =\ 1d$  


So; 16 = d

Multiple Choice

Find the diameter given the radius of 4 when r = 1/2d

• 2

• 6

• 8

Multiple Choice

find the diameter when the radius is 6 using r = 1/2d

• 3

• 8

• 12

Finding Radius when we know diameter

 $d\ =\ 2\left(r\right)$  

Use this formula when determining radius
Diameter is 8, what is the radius?

Diameter = 8

 $8\ =\ 2\left(r\right)$  
do the inverse operation to find value of r

 $\frac{8}{2}\ =\ \frac{2}{2}\left(r\right)$  

4 = 1(r)

so: r = 4

Multiple Choice

Find the radius when given a diameter of 10 using

d = 2r

• 5

• 7

• 12

• 20

Multiple Choice

Find the radius when given a diameter of 14, using d = 2r

• 7

• 12

• 16

• 28

In conclusion how can we easily remember the values of Radius and Diameter?

d = 2r

r = 1/2d

Multiple Choice

Which is correct?

• radius is half the diameter and twice Pi

• radius is half the diameter and diameter is twice Pi

• radius is half the diameter and diameter is twice the radius

• radius and diameter are NOT proportional