What is NOT a real number?
A number which is imaginary, is NOT a real number. All real numbers can be expressed on the number line, while imaginary numbers cannot be expressed on the number line
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I will assume that by "real number" you mean a member of the set of things called "the Real numbers" (As opposed to meaning "what is really a number?") A real number is a number that can be expressed in decimal form. Everything else is not a real number.
Some things that look like they describe numbers do not:
infinity is not a number (not a real number and not a complex number)
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There are many things that are not real numbers. Perhaps the most interesting question is "what numbers are there that are not real numbers?"
(1) Complex numbers.
Some things like Taylor's Theorem behave much better.
(2) Quaternions.
(3) Single complex infinity.
(4) Circle at infinity.
(5) Infinitesimals.
At the other end of the scale, what happens if you try to add infinitely small numbers. Well you can. It's generally a bit messy and does tend to break various things, but it can be useful.
(6) Finite fields.
(7) Rings.
...
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Complex numbers, such as those in the form of ( a + bi ), where ( i ) represents the imaginary unit and ( a ) and ( b ) are real numbers, are not considered real numbers.
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When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.
When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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