this post was submitted on 17 Apr 2024
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[–] bleistift2@feddit.de -2 points 1 year ago* (last edited 1 year ago) (5 children)

This only ever got handed down to us as gospel. Is there a compelling reason why we should accept that (-3) × (-3) = 9?

[–] notabot@lemm.ee 31 points 1 year ago (2 children)

You can look at multiplication as a shorthand for repeated addition, so, for example:

3x3=0 + 3 + 3 + 3 = 9

In other words we have three lots of three. The zero will be handy later...

Next consider:

-3x3 = 0 + -3 + -3 + -3 = -9

Here we have three lots of minus three. So what happens if we instead have minus three lots of three? Instead of adding the threes, we subtract them:

3x-3 = 0 - 3 - 3 - 3 = -9

Finally, what if we want minus three lots of minus three? Subtracting a negative number is the equivalent of adding the positive value:

-3x-3 = 0 - -3 - -3 - -3 = 0 + 3 + 3 + 3 = 9

Do let me know if some of that isn't clear.

[–] bleistift2@feddit.de 13 points 1 year ago* (last edited 1 year ago)

This was very clear. Now that I see it, I realize it’s the same reasoning why x^(-3) is 1/(x^3):

 2 × -3 = -6
 1 × -3 = -3
 0 × -3 =  0
-1 × -3 =  3

Thank you!

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