What is i? What are imaginary numbers?

Squaring numbers always give a positive result or a 0.  For example, 3^2 = 90^2 = 0(-5)^2 = 25.  Using this argument, it follows that we can always get the square root of 0 or any positive number. Now, what about the square root of a negative number, say, -4?

Let’s see:  2 \times 2 = 4 and -2 \times -2 =4; therefore, no number exists that when multiplied by itself equals -4. In fact, it looks like we cannot find the square root of any negative number.

Now, what if — just what if — we invent square root of negative numbers? Nobody would stop us right? We start with -1.  Suppose - 1 has a square root, and we call it i. If \sqrt{-1} = i, then i^2 = -1.  If so, we can also answer the question that we have asked above: \sqrt{-4} = \sqrt{4(-1)} = 2iAs a matter of fact, we can generalize this operation for any negative number.  If a is positive, then -a is a negative number. So, \sqrt{-a} = \sqrt{a(-1)} = i\sqrt{a}.

But what kind of number is i? Surely, it is not a real number since we cannot locate it on the real number line. And, since we have all the real numbers and i is not a  real number, maybe, we should call i an imaginary number.

The Real Number Line

Imaginary numbers were ‘invented’ (or discovered if you prefer) because mathematicians wanted to know if they could think of square root of negative numbers, particularly, the root of the equation x^2 + 1 = 0 (that is, x^2 = - 1 which is the same as  finding the \sqrt{-1}).   Just like the many ‘radical’ ideas in mathematics,   it was not widely accepted at first, but eventually its invention proved to be very useful and has opened a lot of new ideas in mathematics.

In the next post in this series, we are going to discuss about the operations on imaginary numbers. .

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8 thoughts on “What is i? What are imaginary numbers?

  1. Guillermo, this is such a clear post. I have taught about imaginary numbers to my preservice teacher students, and their eyes always glaze over, and I struggled to explain what possible use they had. I look forward to the follow-up post in this series.

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