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Let's solve the given problems step by step:

### a) Diameter of the circle
The diameter \( D \) of a circle is twice the radius \( r \). The formula is:
\[
D = 2 \times r
\]
Given the radius \( r = 6.4 \, \text{cm} \):
\[
D = 2 \times 6.4 = 12.8 \, \text{cm}
\]

### b) Circumference of the circle
The circumference \( C \) of a circle is calculated using the formula:
\[
C = 2 \times \pi \times r
\]
Using \( \pi \approx 3.1416 \) and \( r = 6.4 \, \text{cm} \):
\[
C = 2 \times 3.1416 \times 6.4 = 40.2124 \, \text{cm}
\]
Rounded to 1 decimal place:
\[
C \approx 40.2 \, \text{cm}
\]

### Final Answers:
- **Diameter**: \( 12.8 \, \text{cm} \)
- **Circumference**: \( 40.2 \, \text{cm} \)

Would you like further details or explanations?

Here are 5 related questions:
1. How would you calculate the area of the circle?
2. If the radius were doubled, what would the new circumference be?
3. How does the diameter relate to the area of a circle?
4. What would happen to the circumference if the radius is halved?
5. Can you calculate the area of a sector of this circle with a 90° angle?

**Tip**: Remember, the circumference always increases proportionally to the radius.

The radius of the circle below is 6.4 cm. a) What is the diameter of the circle? b) What is the circumference of the circle? Give your answers to 1 d.p.

Learn how to calculate the diameter and circumference of a circle with a radius of 6.4 cm. Using the formulas for diameter (2 * radius) and circumference (2 * π * radius), this problem is ideal for students in Grades 6-8. Solutions are rounded to 1 decimal place.

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