-9^2 = -81

[insufferablefury]

I need a help from maths teachers.

My dc says her teacher taught the children the answer is 81. And when she has done the work on online maths programme, it's wrong obviously.

I have explained, it's -81, since it's really - (92). And to get answer as 81, the question need to be (-9)2, not -9^2. But being not a maths specialist, cannot explain why in a logical way , and the child blatantly believes the teacher, which is a good thing, but not really if the teacher is wrong.

How do you maths teachers explain this to your students?

Even if you think of it as (0-9), the answer is still 81, it's just more difficult to work out.

This is because in front of every - sign is an invisible zero

That's a really good way of thinking about it.

0-99 you would obviously do the first.

-9*9 it's easy to forget which comes first.

Yes.most people know square of negative numbers are positive. But in this case, it isn't a case of square of negative numbers. If you can't tell the difference of -92 and (-9)2 , I really doubt you have a maths degree.

Calculators are programmed to work using the BODMAS method

Not all of them.

It is madness to teach that -92 = -81. The implication is that in isolation, it is (-9)2

EggysMom, -9 squared is 81. I am not disputing that. But if expressed as -92, I recently found out it's different from (-9)2, which made sense to me that powers/indies takes priority. Basically, the things you are saying is same as what my dc had problem with. I just took it as a fact that the - (minus/negative) was separate from the number 9, so you need to square 9 before multiplying -, while my dc thought - was part of the number.

PurpleDaisies , why is it a madness to teach -9^2 = -81?
When I saw this first time, I thought wtf? But kind of got it. But never really got why. Today, I got an answer, which was simple enough for me to get it. Bodmas. Makes sense. I just don't want my dc to learn it wrong. I even tested to see if what I know is right or wrong, and the calculator says -9^2 = 81. So, that's the right answer.

Relying on a calculator to prove whether an answer is right or not is also madness.

• -9^2 = -81 not 81.

What's -9+3

-6

Surely if you apply bodmas it's -12
If you apply common sense it's -6

Surely if you apply bodmas it's -12

Huh? How does that work?

Addition before subtraction
Add the two together then subtract from zero
But that's madness

Oh, I see what you’ve done.

-(9+3) is not equal to -9 + 3

You don’t have to do addition before subtraction. They can be done in either order.

Addition isn't before subtraction though - they're equal.

But it's the same logic as -9^2
Im on your side, just curious to know what those who think the answer is - 81 think

BODMAS

Tbh I've always found BODMAS a bit crap

PurpleDaisies, I am just a parents of a child , who really have no special knowledge of Maths.
I find it really difficult to dispute you, if you say I am mad to trust calculator, maybe I am.
But I know as a fact, that -9^2 is -81, not 81. If you are saying I am wrong, you need to convince me otherwise.

steath you’ve changed the question.

It might be helpful to see it as
(-9) + (+3)

The minus sign attaches to the number after it.

@StealthPolarBear

It’s -6. Because, as useful as BODMAS is it’s also a bit confusing in that it implies that addition should happen before subtraction (and multiplication before division) when they actual have equal value and should be performed left-to-right.

In your example the minus and plus can be done in either order as there are no brackets so you just read left to right. In OP's example there's both a negative and a power. In which case the power has to come first given there are no brackets because that's what bodmas says. (Autocorrected to vodka )

I know what the answer is :o I also have a maths degree.
I'm just showing that it is equivalently stupid to -9^2 = -81

But I know as a fact, that -9^2 is -81, not 81.

It isn’t!
This is a spectacularly unhelpful way to use BODMAS. There is no “subtraction”. It’s the numeral -9 raised to the power 2. That is 81.

If you’re looking at the question 0 - 92 the answer to that is 81 because 92 = 81. There is a subtraction in that question.

stealth I hadn’t spotted the point you were making-I thought you were a good mathematician! All this nonsense is making me crazy.