简单理解：do well in 具体指某件事做的好，当说擅长某件事的时候两者好像都可以。
Ⅰ. be good at / in 意思接近于 do well in. “在（某方面）出色；擅长……”
be good at / in 强调一种笼统情况，而do well in 可表示一种情况、一次活动中表现出色
be good at 的反义词组为 be poor (weak) at (in).
do well in 的反义词组为 do badly in.
如 ① Mary is good at / in maths. = Mary does well in maths. 玛丽数学很好（指情况）
= 玛丽数学学得很好
② Tom did well in (不宜用be good at.指具体一次)that English test / sports meeting.汤姆在那次英语考试中（运动会中）考得很好（表现出色）
③ Wu Dong does badly in his lessons. = Wu Dong is weak / poor / at / in his lessons.吴冬功课不好
（指情况）
④ Mei Ying did badly in the high jump 梅英在跳高比赛中成绩不好
（具体一次还有不宜替换）
⑤ Mei Ying is weak / poor in / at high jump.梅英不善于跳高
（指笼统情况）Ⅱ. do well 和do badly可单独运用表一种情况；而be good / weak / poor 一定要借助于介词in 或at强调在某一个方面才能。
如 He does well / badly at school. 他在学校里功课很好/很差
