https://www.glc.us.es/~jalonso/calculemus/si-a-b-c-d-f-%e2%88%88-%e2%84%9d-tales-que-a-%e2%89%a4-b-y-c-d-entonces-a-e%e1%b6%9c-f-b-e%e1%b5%88-f/