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.