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Я это запомню.


If the whole context is formal, then "также" doesn't sound too formal.
I don't think "a ещё" could be formal in any context, it's too colloquial.

I like to play tennis. Anya likes to play tennis, too. - Я люблю играть в теннис. Аня тоже любит играть в теннис.
"Аня также любит играть в теннис" sounds formal/unnatural.

I like to go ice...

Indeed.
Don't mix up:
также (also) and так же (the same way)
тоже (too) and то же (the same [thing])

I think examples is the better explanation.

"Также" very often sounds too formal, really formal, and its usage could look strange/unnatural in many contexts.
"Тоже" is not formal at all, but it...