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Original poster

-9^2 = -81

192 replies

insufferablefury · 02/12/2019 18:27

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?

OP posts:
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TheFallenMadonna · 02/12/2019 20:58

I understand your point. I'm saying that asking the teacher to clarify the intention (which is likely to be squaring the negative, given your daughter's answer) is likely to lead to tighter notation in the future.

PurpleDaisies · 02/12/2019 20:58

Really try typing the question on really good calculator, the answer is -81.

It isn’t though. You clearly don’t have brought mathematical knowledge to doubt the infallibility of the answer a calculator gives you.

Teachermaths · 02/12/2019 20:58

It is a very strange question in isolation and not including brackets.

If you put it in a calculator you get - 81 because the calculator does -(9^2).

I'd agree with people saying imagine a 0 at the start.

CharlieWork · 02/12/2019 20:58

OP, I can kind of understand where you are coming from tbh.

The assumption, when faced with -92, is (-9)2, not that you are subtracting from another number.

PurpleDaisies · 02/12/2019 20:56

I should probably say my degree is physics not maths!

Or, it’s the negative of the result of 9 raised to the power 2, which is what the accepted convention of the orders of operations gives you.

I have NEVER seen this. -92 On its own is always (-9)2 by convention.

myduckiscooked · 02/12/2019 20:56

When I was a teenager doing maths calculators would calculate -9^2 as 81 but now the same input on both the Casio and Sanyo scientific non programmable calculators 99% my students use calculate it as -81.

Original poster
insufferablefury · 02/12/2019 20:54

PurpleDaisies, no sorry, I am not wrong. I know it. It's so frustrating to deal with someone who thinks they do know everything. I did respect you too. Really try typing the question on really good calculator, the answer is -81.

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GivenchyDahhling · 02/12/2019 20:53

Nothing like a maths questions to bring out a whole host of maths degrees. I’m sure some of them are real too...

-9^2 as a child’s maths question is super unhelpful for learning unless it’s clear what the purpose is. If it’s an exercise to try and push the point that the square of a negative number is a positive then it’s mathematically incorrect. If it’s an exercise of BODMAS it’s difficult (because of the lack of any value between the - and the 9) but I suppose as an extension question it could be clever.

I suspect the teacher was attempting the former but has insufficient understanding of the maths to form a mathematically correct question.

Original poster
insufferablefury · 02/12/2019 20:50

TheFallenMadonna, no, she made a mistake, she answered the question as 81, and as soon as seen the question, I knew she was wrong. Question was to answer what was a value of -9^2. The answer was -81

OP posts:
TheatreTaxi · 02/12/2019 20:50

If you want to view -9 as a subtraction from zero, the equivalent equation isn't 0 - 9^2.

It's (0-9)^2. Which has to be solved by multiplying out

(0-9)*(0-9)
= (00) + (0-9) + (-90) + (-9-9)
= 0 + 0 + 0 + 81
= 81

PurpleDaisies · 02/12/2019 20:48

How sad that people who are teachers think I am wrong....

You are wrong. You are just wrong.

If you were right, you could square root -81. You can’t without using imaginary numbers.

Original poster
insufferablefury · 02/12/2019 20:47

@TeenPlusTwenties, I don't think I m wrong to think -9^2 = -81. Am I? I assume you have maths knowledge, could you help?

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TheFallenMadonna · 02/12/2019 20:47

Get her to ask whether the intention is to square negative 9 or multiply the square of 9 by negative one. I reckon brackets will appear in subsequent questions.

Confuddledtown · 02/12/2019 20:44

The square of a negative number is always positive. There are no imaginary zeros at the beginning of an expression.

ItsGoingTibiaK · 02/12/2019 20:44

@PurpleDaisies

It’s the numeral -9 raised to the power 2. That is 81.

Or, it’s the negative of the result of 9 raised to the power 2, which is what the accepted convention of the orders of operations gives you.

You could equally consider the use of the minus sign as:

(-1)*9^2

Again, the exponent takes precedence over the multiplication and so the answer is -81.

What people are arguing is that the minus sign doesn’t attach itself to the numeral, but to the result of which ever operation has precedence, by convention.

Original poster
insufferablefury · 02/12/2019 20:43

So, all these people are taking mickey out of me, because I am stupid to think -9^2 = -81. Ok,
How sad that people who are teachers think I am wrong....

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Original poster
insufferablefury · 02/12/2019 20:39

StealthPolarBear, I have seen your past post and kind of respected you. But what you are doing here, no. I had genuine question. And I had great answers. Just because you don't get it, it's pathetic to take a mickey about it. Very disappointed.

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PurpleDaisies · 02/12/2019 20:36

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.

StealthPolarBear · 02/12/2019 20:32

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

titchy · 02/12/2019 20:31

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 Grin)

ItsGoingTibiaK · 02/12/2019 20:30

@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.

PurpleDaisies · 02/12/2019 20:30

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.

Original poster
insufferablefury · 02/12/2019 20:30

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.

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StealthPolarBear · 02/12/2019 20:29

Tbh I've always found BODMAS a bit crap

StealthPolarBear · 02/12/2019 20:29

BODMAS