A(x) =∫x₀ f(x) dx. That long-necked, swan-like symbol is actually a stretched-out S. The S reminds us that a summation is taking place. It’s a summation of a peculiar kind, distinctive to integral calculus, involving a sum of infinitely many infinitesimal strips, all being integrated into a single, coherent area. As a symbol of integration, it’s called an integral sign. Leibniz introduced it in a 1677 manuscript and published it in 1686. It’s calculus’s most recognizable icon. The zero at the bottom of it and the x at the top of it indicate the endpoints of the interval of the x-axis over
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