Draw a circle of radius 3.5 cm. Mark its centre as O. Take a point A on the circle. Join AO and produce it to meet the circle at B.
Assign a special name to AB.

Answer

### Step-by-Step Solution

1. **Gather Materials**: You will need a compass, a ruler, and a pencil.

2. **Draw the Circle**:
- Open the compass to a radius of 3.5 cm using the ruler.
- Place the pointed end of the compass on a paper to mark the center. Label this point as O.
- Keeping the compass at the same width, draw a circle around point O. This circle should have a radius of 3.5 cm.

3. **Mark a Point on the Circle**:
- Choose any point on the circumference of the circle and label it as A.

4. **Join AO**:
- Use a ruler to draw a straight line from point A to point O. This line is the radius of the circle.

5. **Extend AO to Meet the Circle Again**:
- Extend the line AO beyond point O until it meets the circle again. Label this second point of intersection as B.

6. **Identify the Line Segment AB**:
- The line segment AB, which connects points A and B, is a special line in a circle. It is called the **diameter** of the circle.

7. **Calculate the Length of AB**:
- Since AB is the diameter, it is twice the radius of the circle. Therefore, the length of AB is:
\[ AB = 2 \times \text{radius} = 2 \times 3.5 \, \text{cm} = 7 \, \text{cm} \]

### Final Answer
- The diameter AB is 7 cm.

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