Imaginary number's definition misunderstanding

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Discussion Overview

The discussion revolves around the definition and properties of imaginary numbers, particularly the imaginary unit \( i \), where \( i^2 = -1 \). Participants explore the implications of this definition, the relationship between square roots and squares, and the confusion arising from applying standard arithmetic rules to complex numbers.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the relationship between \( i^2 \) and \( \sqrt{-1} \), suggesting that if \( i = \sqrt{-1} \), then \( i^2 \) should equal 1, leading to confusion.
  • Another participant clarifies that \( i^2 = -1 \) is consistent with the definition of \( i \) as \( \sqrt{-1} \) and discusses the implications of squaring and square roots in this context.
  • A different participant points out the ambiguity in the square root operation within complex numbers, indicating that not all properties of real numbers apply to complex numbers.
  • One participant expresses concern about the validity of a property learned regarding the product of radicals, questioning when it can be applied in the context of imaginary numbers.
  • Another participant references a FAQ post that discusses paradoxes arising from misusing arithmetic rules with complex numbers, suggesting further reading for clarification.

Areas of Agreement / Disagreement

Participants express confusion and differing interpretations regarding the properties of imaginary numbers and the application of arithmetic rules. There is no consensus on the resolution of these misunderstandings.

Contextual Notes

The discussion highlights limitations in applying standard arithmetic operations to complex numbers, particularly regarding the square root function and its implications. The ambiguity in definitions and properties of square roots in the complex plane is noted.

alingy1
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Homework Statement



I'm in this self-learning course. I came on this problem I thought of.

So, i^2=-1.
But, isn't i=sqroot of -1?
If so, the product of the two minus -1 and the square root of that should give 1.

Am I not getting something?
I searched the web with the keywords of my question, but I couldn't find any result...
 
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Is i positive or negative?

The relation i^2 = -1 is the converse of saying i = SQRT(-1). If a = SQRT(b), then a^2 = b.

Then, there are these relations:

i = SQRT(-1)
i^2 = -1
i^3 = -i
i^4 = +1
 
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I do not understand what you mean by "The product of two minus -1 and the square of that". Literally interpreted, "two minus -1" is 2- (-1)= 3 but surely that is not what you mean. Did you mean "the product of -1, twice"? Does "that" refer to i? Yes, [itex]i^2= -1[/itex]. The product of "-1, twice" is [itex](-1)^2= 1[/itex]. What does that have to do with [itex]i^2= -1[/itex]?

If there is something you are "not getting", perhaps it is the difference between the "square" and the "square root". The square of -2 is (-1)(-1)= 1. But the square root of -1 is i.

(There is a "technical" point here: while every number has a single square, every complex number has two square roots. The two square roots of -1 are i and -i.)
 
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Oh boy, sorry. I'm a second language speaker so speaking English messes my train of thoughts. :P
Let me repair the situation:
So, I have been taught that:''the product of two radicals is the same as the radical of the product, and vice versa.''-purplemath
But, now, dealing with the imaginary number:
We know that i=sqroot(-1).
So, i^2 should equal 1 since:
i^2=sqroot(-1)xsqroot(-1)=sqroot(-1x-1)=sqroot(1)=1

But, that makes no sense because it is said that i^2=-1

Is the property that I have been taught wrong? If so, when can I not use it?
 
The square root has an ambiguity in the complex numbers. You cannot use all the regular operations possible with positive real numbers.
 
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@alingly1: You may find this post in the FAQ section by micromass interesting:

https://www.physicsforums.com/showthread.php?t=637214

You will find a discussion of some of the "paradoxes" arising from mis-using the usual rules of arithmetic with complex numbers.
 
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