Problem

\(A(r)~=~\pi~(r~+~2)^2\) is the area of a circle with inner radius \(r~\text{length}~\text{units}\) and a uniform border \(2~\text{length}~\text{units}\) wide. Expand the model and interpret the border, physical domain, and output units.

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A(r)=πr²+4πr+4π. The 2 adds uniformly to the inner radius, r≥0, and A is measured in square units.

A(r)=πr²+2πr+4π. The 2 adds uniformly to the inner radius, r≥0, and A is measured in square units.

A(r)=πr²+4πr+2π. The 2 adds to the diameter rather than the radius, r≥0, and A is measured in length units.

A(r)=πr²+4π. The 2 adds uniformly to the inner radius, r may be any real number, and A is measured in square units.

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