Why is 20 not a square root
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Keep in mind that whenever you have a variable inside an expression, you have to think of it as a placeholder, hiding a certain value which we haven't specified yet.
Expressions with variables are a mean to express the generic idea underneath the expression, rather than its particular value.
So, an expression like x-7 represents the idea of subtracting 7 from a particular number. Once you give a specific value to x, the expression will become an actual subtraction between numbers, and you will be able to compute its value.
So, we have several values for x, which are several ways to turn our "abstract" subtraction into an actual, computable subtraction.
If , the expression becomes
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If , the expression becomes
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If , the expression becomes
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If , the expression becomes
and it evaluates to
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If , the expression becomes
and it evaluates to
.