Practice about circles

### 1.1. Solve the exercises in the textbook Practice page 99

**Lesson 1 Textbook page 99**

Find the circumference of a circle with radius r:

a) r = 9m; b) r = 4.4dm; c) r = \(2\frac{1}{2}\)cm

__Solution guide:__

a) Perimeter of the circle is:

9 x 2 x 3.14 = 56.52 (m)

b) The circumference of the circle is:

4.4 x 2 x 3.14 = 27.632 (dm)

c) Convert r = \(2\frac{1}{2}\)cm = 2.5cm

The circumference of the circle is:

2.5 x 2 x 3.14 = 15.7 (cm)

Answer: a) 56.52m;

b) 27.632dm;

c) 15.7cm.

Lesson 2 Textbook page 99

a) Calculate the diameter of a circle with perimeter C = 15.7m.

b) Calculate the radius of a circle with perimeter C = 18.84dm.

__Solution guide:__

a) The diameter of the circle is:

15.7 : 3.14 = 5 (m)

b) The radius of the circle is:

18.84 : 3.14 : 2 = 3 (dm)

Lesson 3 Textbook page 99

The diameter of a bicycle wheel is 0.65m.

a) Calculate the circumference of that wheel.

b) How many meters will the cyclist travel if the wheel rolls on the ground 10 times, 100 turns?

__Solution guide:__

a) The circumference of the wheel is:

0.65 x 3.14 = 2.041 (m)

b) The distance traveled when the wheel rolls on the ground 10 times is:

2.041 x 10 = 20.41 (m)

The distance traveled when the wheel rolls on the ground 100 times is:

2.041 x 100 = 204.1 (m)

Answer: a) 2,041m;

b) 20.41m; 204.1m.

Lesson 4 Textbook page 99

Choose a letter that has a right answer:

Given a semicircle H (see figure). The perimeter H is:

A. 18.84cm B. 9.42cm

C. 24.84cm D. 15.42cm

__Solution guide:__

The circumference of a circle with a diameter of 6cm is:

6 x 3.14 = 18.84 (cm)

The semicircle of the circle is:

18.84 : 2 = 9.42 (cm)

The perimeter of figure H is:

9.42 + 6 = 15.42 (cm)

Choose answer D.

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