If a diameter is 7 ft, what is the radius of the cylinder?

If a diameter is 7 ft, what is the radius of the cylinder?

• A. 3.5 ft
• B. 7 ft
• C. 3 ft
• D. 1.75 ft

Answer: (A)

The main idea is that the radius is half of the diameter. The diameter goes across the circle through the center, while the radius goes from the center to the edge, so d = 2r and r = d/2. With a diameter of 7 ft, the radius is 7 ÷ 2 = 3.5 ft. The other numbers come from misreading the relationship (they would correspond to using the diameter as the radius or dividing by 4).

The main idea is that the radius is half of the diameter. The diameter goes across the circle through the center, while the radius goes from the center to the edge, so d = 2r and r = d/2. With a diameter of 7 ft, the radius is 7 ÷ 2 = 3.5 ft. The other numbers come from misreading the relationship (they would correspond to using the diameter as the radius or dividing by 4).