7 th Grade ALCOS Calculate the circumferences of

7 th Grade ALCOS: Calculate the circumferences of circles.

The circumference of a circle is The distance around a circle Hint: Circumference remember circle around

First we must define some things about a circle. The radius is the distance from the center of a circle to any point on a circle. Hint: I like to define it as halfway through!

The diameter is the distance across a circle through the center.

We use Pi as the measurement to help us find the circumference of a circle. Pi, not Pie!

HINT: Pi =3. 14

Two formulas are used in finding the circumference of a circle. WHEN THE CIRCLE HAS A DIAMETER MEASUREMENT, USE THE FOLLOWING FORMULA. Circumference = d 3. 14 x 4 in. 12. 6 in. 4 in.

When the radius of a circle is given, the following formula should be used. Circumference = 2 r 2 x 3. 14 x 5 cm 31. 4 cm HINT: Always round your answers to the nearest tenth.

NOW…Let’s have some fun. Tell me what formula would be used to solve the next five problems.

3 in C = 2 x 3. 14 x r

8 ft C = 3. 14 d

122 mm C = 2 x 3. 14 x d

17. 5 cm C = 3. 14 x d

122. 5 mm C = 2 x 3. 14 x r

Now that we have determined the correct formula for each, let’s work the problems.

The answer is: 18. 8 in 3 in C = 2 x 3. 14 x r

8 ft C = 3. 14 d The answer is: 25. 1 ft

122 mm C = 2 x 3. 14 x d The answer is: 766. 2 mm

17. 5 cm C = 3. 14 x d The answer is: 55 cm

122. 5 mm C = 2 x 3. 14 x r The answer is: 76, 930 mm