Which formula is the butterfly model?

The basic formula of butterfly model: AD: BC = OA: OC. Butterfly theorem is one of the most wonderful results in ancient Euclidean plane geometry. This proposition first appeared in 18 15 and was proved by W. G. Horner.

The name "Butterfly Theorem" first appeared in the February issue of American Mathematical Monthly (1944), with the title like a butterfly. There are countless proofs of this theorem, math lovers are still studying it, and there are various deformations in the exam.