Mamikon’s theorem, invented by Mamikon Mnatsakanian, is a method in visual calculus that can solve integral calculus problems with ease. Mamikon first came up with this method in 1959 while working on a geometry problem. What’s surprising is that Mamikon’s method does not require any additional information about the dimensions of the object being calculated. It involves an alternate construction of the shape, where a constant-length tangent travels along the shape’s circumference, “sweeping out” the desired area. This method can be applied to various shapes and yields equal areas regardless of the shape’s original form. Mamikon’s theorem has practical applications in finding the area of a cycloid.
https://en.wikipedia.org/wiki/Visual_calculus