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Secondary education

-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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insufferablefury · 02/12/2019 21:19

Thanks TeenPlusTwenties, good to know someone thinks I am not totally crazy.

OP posts:

TryingAndFailing39 · 02/12/2019 21:20

@CharlieWork

-9^2 is definitely 81.

It really isn’t! -9 x -9 = 81 but -92 is -(92) = -(9x9)
(-9)^2 is -9x-9 which of course is 81.
They are not the same.

mummmy2017 · 02/12/2019 21:22

All the YouTube videos say unless the number with the minus is in brackets you do the square then apply the negative number.
So yes answer is -81.
I am old so I got it wrong.

MillicentMartha · 02/12/2019 21:22

Put it into WolframAlpha. -9^2 is -81

mummmy2017 · 02/12/2019 21:26

Bidmas....
There are no brackets.
So you do the square bit next.
Then you apply the negative.

MillicentMartha · 02/12/2019 21:27

There you go...

-9^2 = -81

BigFatLiar · 02/12/2019 21:28

The correct answer is the one that the examining board are looking for either its either minus nine squared or the negative of nine squared. Which one does the examining board say it is? You don't need to agree you just need to do.

If someone showed me -92 I'd think it was minus nine squared rather than the negative of nine squared. If your education board say you ought to think of it as the negative of nine squared then thats what you need to start thinking to get your kids through the exams. Makes it odd if you now have to write minus nine squared as -(92) all the time.

Frogsandsheep · 02/12/2019 21:29

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

It’s not equivalent at all. Everyone knows the + and - in BIDMAS are of equal value. An index is always done before a + or - unless there are brackets. Your examples are not comparable.

MillicentMartha · 02/12/2019 21:29

I love WolframAlpha!

-9^2 = -81

Frogsandsheep · 02/12/2019 21:29

Bold fail!

noblegiraffe · 02/12/2019 21:32

How would I talk about it to a class?

“I have the number ‘minus 9’. I want to square it. This means multiplying it by itself. So if I square ‘minus 9’ I do ‘minus 9 x minus 9’ which is ‘positive 81’. So ‘minus 9’ squared is ‘positive 81’

Remember if you do this on your calculator put brackets around the ‘-9’ so that your calculator knows you’re squaring ‘minus 9’ and you don’t want to subtract 9 squared as that is minus 81 due to BIDMAS.”

Less confusing if you call minus 9 ‘negative 9’.

The calculator rigidly applies BIDMAS and needs the brackets. Kids do not rigidly apply BIDMAS, and would assume that -9^2 was ‘minus 9 squared’, especially if they’d had it hammered into them in the lesson that squaring a negative gives a positive.

Maths tests, teachers and textbooks should really put brackets around the -9 when squaring but can be sloppy. Kids usually don’t.

Technically the answer to -92 is -81, but in a school setting, I’d have guessed that the answer is meant^ to be 81.

TeenPlusTwenties · 02/12/2019 21:37

noble So we're ALL right?

BigFatLiar · 02/12/2019 21:37

Does this imply that negative numbers should be in brackets to distinguish them from positive numbers?

so -9 is assumed to be 0 minus 9 where as (-9) is negative 9?

Fabuleuse · 02/12/2019 21:38

Maths PhD here popping in. An actual, genuine one, in pure maths. Sorry don't have time to type out a proper explanation (sick kids), essentially BiDMAS or whatever acronym you prefer as convention is exponent attaches itself to what immediately precedes it, but -9^2 is definitely -81. I'm quite shocked that anyone with a maths or physics degree would think otherwise tbh!

MarshaBradyo · 02/12/2019 21:38

Just asked ds age 9 this he said 81. We did it on my iPhone calculator and a normal one and got -81 and 81.

He’s going to ask his maths teacher tomorrow.

mummmy2017 · 02/12/2019 21:44

I passed Maths , O level before bidmas, if an exam board wanted you to do something first they put brackets round it.
I actually think computers have made maths sloppy.
Maybe it is time they were made compulsory in exams.

ItsGoingTibiaK · 02/12/2019 21:45

@PurpleDaisies Interesting that you have a physics degree. I do, too, but mine taught me to be open-minded and curious, and to look at things from alternative angles, rather than to tell people they 'are just wrong'.

If you bother to look online, you'll see there's lots and lots of discussion about this very topic, with an acceptance that there will never be a entirely definitive answer - but the general consensus is that the OP is correct.

Out of interest, how would you treat the coefficient -x2 if it were the leading term in a polynomial? As -(x2) or as (-x)^2 ? And do you think the unary operator is treated differently in an algebraic expression than in a purely numerical one?

BigFatLiar · 02/12/2019 21:46

As an equation its simply x*2 where x is -9 this give -81?

Utility · 02/12/2019 21:46

mathcentral.uregina.ca/QQ/database/QQ.09.09/h/andrea1.html

BigFatLiar · 02/12/2019 21:46

sorry x^2 not x*2

TryingAndFailing39 · 02/12/2019 21:50

I'm quite shocked that anyone with a maths or physics degree would think otherwise tbh!

I know and a number of these mathematicians have been quite sneery towards OP despite being wrong!

TeenPlusTwenties · 02/12/2019 21:51

BigFat no. x2 where x=-9 means (-9)2 which is 81 (because a negative times a negative is a positive).

The argument discussion is about whether -92 equals -(92) or (-9)^2 as the former gives -81 whereas the latter gives 81.

I think noble gave a fair summary of why -81 is correct, but that people often really mean (-9)^2 (ie 81) and so the intention should be clarified.

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insufferablefury · 02/12/2019 21:52

Noblegiraffe, but the answer was -81. and she was wrong by answering 81. I don't think it was meant to be 81, since the teacher only set the questions online using the maths website, she didn't write the questions herself.
Whole point of my thread was to know how to explain that it's -81, not 81. My dd is very able at maths, but has asd. So tend to believe what the teacher says literally. Difficult to change when she has learned negative times negative = positive in class, but got totally separate set work to do at home.

OP posts:

TryingAndFailing39 · 02/12/2019 21:54

For the expression x2, if x= -9 then you’d put brackets in to get (-9)2=81. I’ve recently done that with year 7 and when you substitute in you’d put the brackets in

TryingAndFailing39 · 02/12/2019 21:55

Cross post with @TeenPlusTwenties

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