Various branches of mathematics have mathematical beauty. Some of this are visual, such as the mandelbrot set, while others are logically sublime, such as the recursive simplicities of peano arithmetic and surreal numbers. What are some examples of mathematical beauty in school mathematics. I am mostly intrested in any examples for High School, but any other K12 examples would be appreciated as well.
Also, I am not asking for things marginally related to a school subject, but something that either helps a student understand or reinforce an existing understanding of a mathematical concept by making it aesthetically pleasing.