6th Grade Math Textbook, Fundamentals

Lesson 10-8 for exercise sets. &KDSWHU 

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Find the area or the circumference of the circle in terms of .

1.d28 mm; A? 2.r82.3 km; C? 3.r100 yd; A?

For exercises 4–9, use your calculator’s key. Round to the nearest whole unit.

Find the area and the circumference of the circle.

4.r16 mm 5.d55 km 6.r100 yards

Find the unknown measurement.

7.C20 in.; d? 8.A200.96 cm^2 ; r? 9.C62.8 ft.; d?

10.Discuss and Write If you know the circumference of a circle, can you find its area? Explain.

A round garden has an area of 154 ft^2. Find its diameter.

Use the key on your calculator. Round to the nearest whole number.

Ar^2 Use the formula for area of a circle.

154 r^2 Substitute 154 for the area of the circle.

154  r^2   Divide both sides by .

 Use your calculator to take the square root of both sides.

7.0014... r Round to the nearest whole number.

7 r

The radius is about 7 ft, so the diameter is 2(7), or about 14 ft.

154  r^2

1

Find the area of a circle with a radius of 3.5 inches. Use the key

on your calculator, and round to the nearest tenth. The circle on the

grid at the right can help you approximate the area before you do

the calculations. (Assume each square in the grid represents 1 in.^2 )

Ar^2 A(3.5)^2 A12.25 A38.5 in.^2

The circle’s area is approximately 38.5 in.^2

If you know the circumference or area of a circle, you can use algebra

to find its radius or diameter.

The circumference of the circular face of a wristwatch is 70 mm.

Use the circumference formula and your calculator’s key

to find the maximum length of the minute hand (the radius

of the circle). Round your answer to the nearest hundredth.

C 2 r

70  2 r Substitute 70 for C.

 Divide both sides by 2to isolate r.

11.14 r Simplify.

The maximum length of the watch’s minute hand is approximately 11.14 mm.

2 r

2 

70

2