Area of Circles and Semicircles

Circles: Finding the Area & Circumference

Applying the formulas for circumference and area of a circle to semi-circles.

Parts of Circle (quick review)

• Circle is a set of all points that are equidistant from another single point called the center.

• r = radius d = diameter

• Diameter: d = 2r  or   d = 2・r

• Radius: r =  $\frac{d}{2}$  or   r = d ➗ 2

Multiple Choice

What is the diameter of this circle?

• 3.7 m

• 74 m

• 4 m

• 7.4 m

Multiple Choice

What is the radius of this circle?

• 13 ft

• 6.5 ft

• 26 ft

• 7.5 ft

Multiple Choice

If the the diameter of the large circle is 36 cm, what is the radius of each of the small circles?

• 12 cm

• 24 cm

• 18 cm

• 6 cm

Finding Circumference & Area

• Circumference = π × Diameter or C =πD

    or  C=π2r or C=2πr

• Area = π × radius x radius   or   $Area\ =\ \pi\ x\ r^2\ \ or\ \ A=\pi r^2$   
    

Multiple Choice

I measured from the center of the wheel to the outside edge and it was 62.2 cm. What is the circumference of the wheel? Use 3.14 for pi.

• 62.2 cm

• 124.4 cm

• 195.303 cm

• 390.616 cm

Multiple Choice

I wrapped a tape measure around the trunk of the tree and it was 14.13 feet. What is the diameter of the tree? Use 3.14 for pi. Round to the nearest hundredth.

• 2.25 ft

• 44.37 ft

• 4.5 ft

• 17. 27 ft

Multiple Choice

What is the area of this semicircle if the diameter is 8 cm? Use 3.14 for pi.

• 25.12 sq cm

• 50.24 sq cm

• 200.96 sq cm

• 100.48 sq cm

Multiple Choice

What is the circumference of this semicircle if the diameter is 8 cm? Use 3.14 for pi.

• 25.12 cm

• 50.24 cm

• 20.56 cm

• 12.56 cm

Multiple Choice

A circular apple pie has an area of 706.5 cm². What is the radius of the pie? Use 3.14 for pi.

• 225 cm

• 15 cm

• 112.5 cm

• 30 cm

Poll

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