Sixty years ago, the math teacher who taught partial differential dictated: "dui." I don't know why I didn't read it. I heard it was French mathematician Legendre, Adrian-Marie; ; 1752 ~ 1833 first introduced symbol? Read in French and read in English.

Maybe he changed the capital of Greek delta to "? The appearance of ". After the apprentice, step by step.

I forgot all about it except this one. Confucius said, "Can it be passed on?" Want to be ashamed!