A Mathematical Cipher [cIosed]

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I'm just learning to use $$\mathrm{\LaTeX}$$, and I need practice, so I made this cipher puzzle.

Code:

$$\dfrac{1}{16}\sqrt{s(18}.00\sqrt{-1.5}\ \ \ \ \dfrac{1}{10}\sqrt{16}\dfrac{1}{2}\dfrac{1}{10}\ \ \ \ \dfrac{1}{15750}\sqrt{xy18}\dfrac{1}{2}\ \ \ \ \dfrac{1}{10}\begin{matrix}n\\k\end{matrix}\Box2)\!\!\overline{\ \ 21}\dfrac{1}{2}\ \ \ \$$ $$\overline{)}\dfrac{1}{18}4)\!\!\overline{\ \ 12}2)\!\!\overline{\ \ 21}\sqrt{5}\overline{\Large{3}}\overline{.5}\overline{\Large{3}}\overline{.5}\Box\ \ \ \ \dfrac{1}{16}\overset{\uparrow}{\bigodot}\dfrac{1}{2}\ \ \ \ \overline{\Large{3}}\dfrac{1}{100}\ \ \ \ \overline{\Large{3}}\dfrac{1}{10}\ \ \ \ \overline{)(}2)\!\!\overline{\ \ 21\!500}\dfrac{1}{10}\overline{s-b}\Box$$

Question: Solve this cipher and find the hidden three-word phrase.

Sorry for my low-quality cryptic-clue...

Hint 1:

Diacritics should be ignored after you cracked the cipher.

Hint 2:

You may have noticed the tag, and you may be asking for the enumeration.
Well, here it is!
(k,i,j)

Hint 3:

What? An enumeration with letters instead of numbers?
Just simply encode the letters (not the punctuation) using the same key
and remove all diacritics.

Hint 4:

The hidden three-word phrase may often appear in some school exercise books.

• I seem to have found a loophole in the rules. – Scratch---Cat Aug 18 '20 at 8:54
• I fear your '[closed]' joke in the title may well be putting a lot of people off even clicking on your puzzle to read it, let alone trying to solve it! ;-) – Stiv Aug 18 '20 at 12:12
• @Steve : Not a second one in the series. I came up with my puzzle all by myself. – Scratch---Cat Aug 23 '20 at 11:00
• @Stiv we have direct evidence that at least 13 people (at time of writing) have clicked on it and read as far as your comment... whether they're people who have even the slightest clue how to start solving it is another matter of course. – Steve Aug 28 '20 at 14:21
• No answer for months? Wait, was my previous comment deleted? @Stiv – Scratch---Cat Nov 14 '20 at 10:50