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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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StealthPolarBear · 02/12/2019 20:29

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

titchy · 02/12/2019 20:28

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

PurpleDaisies · 02/12/2019 20:28

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

PurpleDaisies · 02/12/2019 20:27

Oh, I see what you’ve done.

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

StealthPolarBear · 02/12/2019 20:27

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

PurpleDaisies · 02/12/2019 20:26

Surely if you apply bodmas it's -12

Huh? How does that work?

StealthPolarBear · 02/12/2019 20:26

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

PurpleDaisies · 02/12/2019 20:25

-6

StealthPolarBear · 02/12/2019 20:25

What's -9+3

Original poster
insufferablefury · 02/12/2019 20:23
  • -9^2 = -81 not 81.
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PurpleDaisies · 02/12/2019 20:22

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

Original poster
insufferablefury · 02/12/2019 20:21

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.

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Original poster
insufferablefury · 02/12/2019 20:10

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.

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PurpleDaisies · 02/12/2019 20:04

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

Original poster
insufferablefury · 02/12/2019 20:01

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.

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ShouldI101 · 02/12/2019 19:59

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.

CharlieWork · 02/12/2019 19:58

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

TeenPlusTwenties · 02/12/2019 19:57

But Charlie

Say you have +10 - x^2

and x=9, that's 10-81 = -71. yes?

So -x2 + 10 has to come to -71 too. So -x2 means -(x2) not (-x)2

EggysMom · 02/12/2019 19:55

I did Maths many many many years ago, and we never had the "presume there is a zero at the start of the expression". So whilst I appreciate that BODMAS says the ordinal comes first, tome the '-' is not a minus sign (as in 0-9). I don't know the correct term but it's an indicator that the first number in the expression is -9 and not +9. It belongs to the 9, rather than standing alone. So to me, the equation is -9 squared which is +81.

CharlieWork · 02/12/2019 19:51

Calculators are programmed to work using the BODMAS method

CharlieWork · 02/12/2019 19:49

I have used a scientific calculator and have a maths degree! The square of a negative number is positive.

I have read the full thread, thank you

Original poster
insufferablefury · 02/12/2019 19:46

CharlieWork, no you are wrong. It's not -9 x -9. It's - (9^2).= - 1 x 81
Try using scientific calculator. By typing in -92, and typing in (-9)2 , you get different answers.

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TeenPlusTwenties · 02/12/2019 19:44

Charlie RTFT.

-9 x -9 = (-9)2 not -92

Stillmuddlingthrough · 02/12/2019 19:43

I wouldn’t assume a calculator works on the correct order of operations - many don’t especially non-scientific ones. It is definitely -81. Maths grad and post grad plus maths job but also the correct answer of -81 is exactly what my primary and secondary school children are taught.

CharlieWork · 02/12/2019 19:31

-9^2 is definitely 81. Not negative. You can check by plugging into a calculator.

It is just -9 x -9 = 81

If it was -(-9^2), then it would be -81