Calculate the circle circumfrance's upper bound if the area is 400cm(square)
\(C = 2 \pi r\\ A = \pi r^2 = \dfrac{C^2}{4\pi}\\ C= \sqrt{4\pi A} = \sqrt{1600\pi}=40\sqrt{\pi}~cm\)
That is the maximum....actually, the ONLY C for a circle with area = 400 cm^2