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Curve-Fitting the Arch

The Gateway Arch in St. Louis was built as a monument to the westward expansion of the United States. It is 630 feet in height, making it the tallest man-made monument in the United States.

Task 1: The Arch is probably the world's largest "real" optical illusion, because despite visual conclusions, it is exactly as wide as it is tall. Check it out by measuring on the picture.

Task 2: Is the Arch a parabola? How could you determine an answer to this perplexing question? Make some measurements...and what are your conclusions? And if it is not a parabola, then what shape is it?

NOTE: One reader (J.S., WA) suggests that a reasonable formula is f(x) = - (1/157.5) x2 + 630. Agree or disagree?