Same reason that a double negative makes a positive.
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bleistift2@feddit.de 8 months ago
This only every got handed down to us as gospel. Is there a compelling reason why we should accept that (-3) × (-3) = 9?
Davel23@fedia.io 8 months ago
ImplyingImplications@lemmy.ca 8 months ago
Here’s a other example:
A) -3 × (-3 + 3) = ?
You can solve this by figuring out the brackets first. -3 × 0 = 0
You can also solve this using the distributive property of multiplication, rewriting the equation as
A) -3 × (-3 + 3) = 0 (-3 × -3) + (-3 × 3) = 0 (-3 × -3) - 9 = 0 (-3 × -3) = 9
If (-3 × -3) didn’t equal 9 then you’d get different answers to equation A depending on what method you used to solve it.
notabot@lemm.ee 8 months ago
You can look at multiplication as a shorthand for repeated addition, so, for example:
In other words we have three lots of three. The zero will be handy later…
Next consider:
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:
Finally, what if we want minus three lots of minus three? Subtracting a negative number is the equivalent of adding the positive value:
Do let me know if some of that isn’t clear.
bleistift2@feddit.de 8 months ago
This was very clear. Now that I see it, I realize it’s the same reasoning why x^(-3) is 1/(x^3):
affiliate@lemmy.world 8 months ago
i think this is a really clean explanation of why (-3) * (-3) should equal
9
. i wanted to point out that with a little more work, it’s possible to see why (-3) * (-3) must equal 9. and this is basically a consequence of the distributive law:the first equality uses
0 * anything = 0
. the second equality uses(3 + -3) = 0
. the third equality uses the distribute law, and the fourth equality uses3 * (-3) = -9
, which was shown in the previous comment.so, by adding
9
to both sides, we get:9 = 9 - 9 + (-3) * (-3).
in other words,
9 = (-3) * (-3)
. this basically says that if we want the distribute law to be true, then we need to have (-3) * (-3) = 9.it’s also worth mentioning that this is a specific instance of a proof that shows
(-a) * (-b) = a * b
is true for arbitrary rings. (a ring is basically a fancy name for a structure with addition and distribute multiplication.) so, any time you want to have any kind of multiplication that satisfies the distribute law, you need (-a) * (-b) = a * b.in particular,
(-A) * (-B) = A * B
is also true whenA
andB
are matrices. and you can prove this using the same argument that was used above.