A basic question of basic writing

Question

When we define some term by a notation starting with the word "define" or "let", which of the following is correct?

Define $A=x+2y$; or, define $A \equiv x+2y$.

Answer 1

Yes, many sign than look like equation is usually used for definitions:

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Equivalence $\equiv$, also used for congruence in geometry.
Assignment signs =:, :=, used is computer science to indicate assignment
Logical equivalence :⇔
Define sign ≜, usually used in physics
And the most obvious: ≝ , (I personally recommend)

But it is not necessary if you write the world "define" before, just use $=$