도 has some meanings case by case.
1. Something was already included, and you wanna Include another. (also or too)

나도 갈래
-> I'll go, too.
우리는 밥을 먹고 영화도 봤어.
-> We had dinner, and also see a movie.

2. Because you yield extreme case, other case is included. (Even)
어린애라도 대답할 수 있다.
-> Even a child can answer it.
[explanation] You yield level of subject, so people who is older than child can answer it.

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In that case, 고 is the meaning of AND.
When you wanna connect with adjectives, you can use 고.

그녀는 똑똑하고 이뻐요.
->She is smart and pretty.

나는 슬프고 아팠다.
-> I was sad and sick.