굳이 말하면 겉넓이의 변화량이 되지 않을까
https://math.stackexchange.com/questions/2216646/physical-significance-of-8-pi-r-the-second-derivative-of-the-volume-of-a-sphe
굳이 말하면 겉넓이의 변화량이 되지 않을까
https://math.stackexchange.com/questions/2216646/physical-significance-of-8-pi-r-the-second-derivative-of-the-volume-of-a-sphe