Which formula gives the diameter in terms of the radius?

Which formula gives the diameter in terms of the radius?

• A. d = πr
• B. d = r/2
• C. d = 2r
• D. d^2 = 4r

Answer: (C)

The diameter is twice the radius. In a circle, the radius goes from the center to the edge, and the diameter runs straight across from one edge to the opposite edge, passing through the center. That means the diameter is made up of two radii, so d = 2r. For example, if the radius is 3 units, the diameter is 6 units. The other forms don’t fit this straight-line, across-the-circle relationship: d = πr would imply a π factor that doesn’t describe a diameter; d = r/2 would give a diameter smaller than the radius; and d^2 = 4r introduces a squared term that isn’t part of the simple diameter–radius link.

The diameter is twice the radius. In a circle, the radius goes from the center to the edge, and the diameter runs straight across from one edge to the opposite edge, passing through the center. That means the diameter is made up of two radii, so d = 2r. For example, if the radius is 3 units, the diameter is 6 units.

The other forms don’t fit this straight-line, across-the-circle relationship: d = πr would imply a π factor that doesn’t describe a diameter; d = r/2 would give a diameter smaller than the radius; and d^2 = 4r introduces a squared term that isn’t part of the simple diameter–radius link.