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Original poster

How do I explain to a Y7 why when you multiply two negative numbers it becomes a positive?

135 replies

loveyouradvice · 04/07/2025 22:07

Just that really - I know it does, but can't explain why!

Hoping @noblegiraffe and others might know!!

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BathsAreBliss · 04/07/2025 22:44

An enemies enemy is my friend (- x - = +)
a friends enemy is my enemy (+ x - = -)
an enemies friend is my enemy (- x + = -)
a friends friend is my friend ( + x + = +)

I appreciate this doesn’t help with “why” but I’ve been a qualified accountant for 10+ years and I recite this often 😂

Love51 · 04/07/2025 22:44

Are they struggling to understand or remember? find it helps to take it down to using plus or minus one as one of the numbers.
-32x1 is obviously -32
So -32x-1 is +32.
What else could it be? You know it isn't -32 because that's - 32x1.

Tryingtokeepgoing · 04/07/2025 22:44

I was taught, rightly or wrongly, that because a negative was the opposite to a positive, if you multiplied a negative by another negative you were applying two opposite transactions consecutively, which meant you ended up back with a positive. Effectively, one negative took you through 180 degrees, and another negative took you another 180 degrees back to a positive. But, despite having an O level, AO level and A levels in maths and further maths that’s the best I can manage!

modgepodge · 04/07/2025 22:39

@RunsWithDinosaurs
Some of it is though…like why 65 rounds to 70 not 60, and why we do the multiplication first in 6+4x5, for example. It’s just conventions of how we record stuff rather than inherent maths I think…

the explanation involving the baby walking is the best explanation I’ve seen for multiplying negative numbers. I too was taught to think of 2 - signs making a + but that doesn’t actually explain WHY. And I like to know why!

NotEnoughRoom · 04/07/2025 22:39

noblegiraffe · 04/07/2025 22:19

There's a basic explanation where you fill out a times tables grid and just continue the patterns backwards, but I like this explanation of it being like a child walking backwards on the number line while facing in the negative direction.

OMG I love the baby explanation, and I’m definitely going to need it to explain to my DD soon.

thanks @noblegiraffe !!

HeddaGarbled · 04/07/2025 22:35

I know. But this particular rule ……..

RunsWithDinosaurs · 04/07/2025 22:22

HeddaGarbled · 04/07/2025 22:20

I don’t think there is a reason: it’s just a made up rule. Interested to see if anyone comes up with anything.

What?!? Maths isn't just arbitrary rules made up to confuse school children and test their memories!!!

HeddaGarbled · 04/07/2025 22:20

I don’t think there is a reason: it’s just a made up rule. Interested to see if anyone comes up with anything.

noblegiraffe · 04/07/2025 22:19

There's a basic explanation where you fill out a times tables grid and just continue the patterns backwards, but I like this explanation of it being like a child walking backwards on the number line while facing in the negative direction.

How do I explain to a Y7 why when you multiply two negative numbers it becomes a positive?
How do I explain to a Y7 why when you multiply two negative numbers it becomes a positive?
How do I explain to a Y7 why when you multiply two negative numbers it becomes a positive?
ASongbirdAndAOldHat · 04/07/2025 22:16

Embarrassingly I have no idea, but we remember it as - and a - put together is a +

I have tried to understand it, maybe someone will be able to finally explain it on here.