What formula is used to calculate the area of a round pipe?

The formula used to calculate the area of a round pipe, specifically the cross-sectional area, is derived from the formula for the area of a circle. The area ( A ) of a circle is given by ( A = \pi r^2 ), where ( r ) is the radius. This is why the formula provided accurately represents the calculation needed for the cross-sectional area of a round pipe.

In context, using radius instead of diameter is important because the radius is the distance from the center of the circle to its edge, while the diameter is twice the radius. Option A correctly applies this understanding by using the radius to compute the area directly.

Other options provide incorrect methods or calculations that do not match the standard formula for a circular area. For example, one option multiplies radius and height, which pertains to volume rather than area, while another uses diameter directly without correctly factoring the radius, and yet another involves an unnecessary halving of the diameter, which complicates the understanding of the radius in the area formula. Therefore, recognizing that the area is fundamentally based on the radius squared multiplied by pi is crucial for accurate calculation.