CirclesCircles Objective: To find the circumference and area of a circle.

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

Diameter Radius center What is the formula relating the circumference to the diameter?

An approximation to π π≈ forever….

Two formulas are used in finding the circumference of a circle. WHEN THE CIRCLE HAS A DIAMETER MEASUREMENT, USE THE FOLLOWING FORMULA. C = d When the radius of a circle is given, the following formula should be used. C= 2 r

Circumference = d 3.14 x 4in. 12.6in. 4in. EXAMPLE #1 FIND THE CIRCUMFERENCE OF THE CIRCLE.

Circumference = 2 r 2 x 3.14 x 5cm 31.4cm HINT: Always round your answers to the nearest tenth. 5 cm EXAMPLE #2 FIND THE CIRCUMFERENCE OF THE CIRCLE.

Circumference (It Just Makes Sense) Chorus If I need to measure up a clock (circumference) Measure the post on a dock (circumference) If I’m buying rims for a car (circumference) Hey, yo, 2πr! (It just makes sense) If I need to measure up a hat (circumference) Spinnin’ records like that (circumference) If I need to pick the right jar (circumference) Hey, yo, 2πr! (It just makes sense) Verse I Check out and see all the round things you can find So many circles out there, it will blow your mind It can be hard to measure distance around But once you use a little math you can figure it out If you need to find the distance around a circle shape All you need is one formula and a measuring tape Then you measure up the radius so you can figure out ‘Cause twice that radius times π is what a circle’s all about Chorus Verse II You know that every circle—if it’s big or little— Has one single point that’s right in the middle So you start on one edge, draw a straight line Right through the center and to the other side That’s the diameter. And half of that is the radius That’s the crucial number that we need to figure this out Then we multiply it by 2π Circumference, 2πr, there’s no need to ask why

PRACTICE #1 1. Find the circumference. 8 ft. 2. Find the circumference of a circle with a radius of 5 cm.

AREA FILLING THE CIRCLE. A =  r²

EXAMPLE #1 Find the area of the circle. ● 2 in A =  r² A = 3.14(2) ² A = 3.14(4) A = in ²

EXAMPLE #2 Find the area of the circle. ● 20 cm A =  r² A = 3.14(10)² A = 3.14(100) A = 314 cm ²

PRACTICE #2 Find the area of the following circles. ● ● A =  r² A =3.14 (1.5) ² 3 yd 7.5 mi A = 3.14 (7.5)² A = 3.14 (2.25) A = 3.14 (56.25) A = 7.1 yd ² A = mi² 1. 2.

Area and Circumference of a Circle Song This song is sung to the tune of the Wheels on the Bus The area of a circle is Pi r squared Pi r squared Pi r squared The area of a circle is Pi r squared All through the circles The circumference of a circle is 2 pi r 2 pi r 2 pi r The circumference of a circle is 2 pi r All around the circle