Circle, center of circle, diameter, radius, Draw circle,

### 1.1. About circle

**Comment :** In a circle

- The center O is the midpoint of the diameter AB.
- The length of the diameter is twice the length of the radius.

### 1.2. Draw a circle

Use a compass to draw a circle with a radius of 2cm.

### 1.3. Solve the textbook exercises page 111

**Lesson 1:** Name the radius and diameter of each circle:

__Solution guide:__

- Radius: The line segment connecting the center to a point on the circle.
- Diameter: The line segment passing through the center, connecting two points lying on the circle.

a) Radius: OM, ON, OP, OQ.

Diameter: MN, PQ.

b) Radius: OA, OB.

Diameter: AB.

**Lesson 2:** Please draw a circle with

a) Center O, radius 2cm

b) Center I, radius 3cm.

__Solution guide:__

Use a compass to draw a circle:

- Open the compass a distance equal to the radius of the circle to be drawn.
- Mark a fixed point on the notebook, name it and call it the center.
- Place one fixed leg of the compass with the center, keep it fixed and rotate the other leg once, from which you get a circle with arbitrary center and radius.

**Lesson 3:**

a) Draw radius OM, diameter CD of the following circle:

b) Which sentence is true and which is false?

– The length of the line segment OC is longer than the length of the line segment OD.

– The length of line segment OC is shorter than the length of line segment OM.

– The length of line segment OC is half the length of line segment CD.

__Solution guide:__

- Mark any point M on the circle, connect O to M. Through O draw a line, cut the circle at two points, name it C and D.
- Using knowledge: In a circle, the radii are of equal length and the length of the diameter is twice the length of the radius.

a)

b)

– The length of the line segment OC is longer than the length of the line segment OD (False, because OC and OD are both radii of a circle, they have the same length)

– Length of line segment OC is shorter than length of line segment OM (False)

– The length of the line segment OC is half of the length of the line segment CD (Correct because in a circle the radius length is half the length of the diameter).

### 1.4. Textbook exercise solution page 112

**Lesson 1:** Draw a picture according to the following steps

*Step 1:* Draw a circle with center O and radius OA.

*Step 2: *Draw a circle decoration (center A, radius AC; center B, radius BC).

*Step 3:* Draw a circle decoration (center C, radius CA; center D, radius DA).

**Lesson 2:** Color the picture drawn in lesson 1

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