There are those like the ones in Singapore, which teach some arithmetic or algebra and some geometry each year with connections among them used.
Similar programs exist in many other countries.
Singapore is mentioned because textbooks in that country are good, are written in English, and are accessible in the United States. These have a different focus, trying to teach mathematics in the context of real-world problems. It is this second type of integrated program that has become the focus of controversy in the United States. Nothing is written in the textbook about where this formula comes from, but the following information is given on page T313 in the Teacher's Guide. This will be followed up a bit in a later probability unit in Course 2.