[From the irrelevant anecdote desk] Just a note to say this is how we used to calculate land area where I live!

Nowadays there are smart-phone apps which use GPS to calculate area automatically, but when plots of land in the village traded hands two decades ago a low-tech procedure was used:

Approximate the plot of land as a quadrilateral. Measure the sides a, b, c, d. Multiply (a+c) by (b+d) to get the area. In the example this would be (1+1)×(1+1) = 4.

Explanation: the lengths were measured in *wa*; the resulting area is in square meters. *Wa* is an ancient unit of length which has since been standardized as 2.0 meters.

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NETA: The “paradox” of Gabriel’s horn (also called Torricelli’s trumpet) attracted great interest from mathematicians and philosophers when it was discovered almost 400 years ago. Its inventor, Evangelista Torricelli, famous for his textbook about possible pitfalls in calculus, spent considerable effort trying to find the (non-existent) flaw in his calculation of its surface area.