Advanced search

-9^2 = -81

(193 Posts)
insufferablefury Mon 02-Dec-19 18:27:57

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 - (9^2). 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’s posts: |
StealthPolarBear Mon 02-Dec-19 18:28:53

(-9)^2 is 81

StealthPolarBear Mon 02-Dec-19 18:29:48

Sorry I see you're agreeing. You wouldn't usually need to use brackets. The square of a negative is positive. Convention is that brackets are assumed.

GuyFawkesDay Mon 02-Dec-19 18:31:22

-9 squared it 81. Squaring gets ride of the negative

Teachermaths Mon 02-Dec-19 18:33:45

It depends on the question....

Is it substituting - 9 into an expression?

Or is it just - 9^2 as a standalone?

Xiaoxiong Mon 02-Dec-19 18:40:15

The convention is that exponents take precedence over negation. So if I see -3^2, with no brackets, I do the square first, taking care of the exponent, then I make it negative.


This is because in front of every - sign is an invisible zero. So the first sum above is "actually" 0-3^2, which hopefully is more obviously equal to -9 rather than 9.

insufferablefury Mon 02-Dec-19 18:40:55

StealthPolarBear, I know we normally don't need to use brackets. The question we got stuck was -9^2, so I used brackets to show my dc the difference in -(9^2) and (-9)^2. The rule of square of negative is positive. I know that. But it need to be (-9)^2 or -9 x -9, instead of -9^2. to get 81. Seems to be very confusing when teacher says the answer for -9^2 is 81, but really, -9^2 = -81. And I can't explain well, the difference of having - in front of the number with powers.

OP’s posts: |
ItsGoingTibiaK Mon 02-Dec-19 18:41:49

It's a BODMAS (or PEDMAS or BIDMAS) thing. Squaring is an ORDER or EXPONENT, which means it is operated before the minus sign.

Xiaoxiong Mon 02-Dec-19 18:42:11

Oh fucking hell. All my carats made italics.
-3 ^ 2 = - 9. Because there is an invisible 0 in front of the negative number.

StealthPolarBear Mon 02-Dec-19 18:42:41

But thay is the convention. I don't understand why you're insisting it needs brackets.
A negative number squared is shown by -x2 where the 2 is superscript.

insufferablefury Mon 02-Dec-19 18:42:50

Teachermaths, it's stand alone. So the answer is definitely -81.
-9^2 = ?

OP’s posts: |
insufferablefury Mon 02-Dec-19 18:45:00

ItsGoingTibiaK , that totally makes sense! Great. I think I got an answer how to explain. Thank you!

OP’s posts: |
insufferablefury Mon 02-Dec-19 18:48:58

And thank you, Xiaoxiong. It's too complicated to understand the thing about 0 for me, but now I got it, that exponents take precedence over negation. I think I can explain properly now why. Thank you.

OP’s posts: |
hiddenmnetter Mon 02-Dec-19 18:49:01

The answer is due to the order of operations. BODMAS- brackets, orders, divisions, multiplications, additions, subtractions.

The exponent is an order, so the 9 is raised to its square of 81, and then either subtracted or multiplied by -1, whichever you use. -9 x -9 = 81 though.

This is why the quadratic equation is +/- at the beginning, because both the positive and negative answer are right (and why quadratic formulae make parabolas)

hiddenmnetter Mon 02-Dec-19 19:07:39

I see I've cross postedfblush

TryingAndFailing39 Mon 02-Dec-19 19:10:06

It’s definitely -81 with no brackets. Brackets are needed to square the -9 and make the answer 81.

insufferablefury Mon 02-Dec-19 19:19:47

Thank you hiddenmnetter.
And thanks, everyone. I explained Bodmas/Pedmas and I think(hope) dc understood. Says so. And I asked her to ask the teacher if still not sure.
Funnily enough, I am leaning maths myself, and have encountered this question asked by someone else recently, and being an adult, I just accepted this as a fact, that was an only reason I knew -9^2 was -81. But don't have enough knowledge to explain it to someone else.
It helped me understand too. So, thank you.

OP’s posts: |
myduckiscooked Mon 02-Dec-19 19:23:58

Constant issue with my students making a mistake on this one. You are absolutely correct.

(-9)^2 is in words - minus mine all to be squared.

-9^2 is simply minus nine squared

The addition of brackets means that everything within the bracket is squared. Omit the brackets and you are only squaring the nine.

CharlieWork Mon 02-Dec-19 19:31:45

-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

Stillmuddlingthrough Mon 02-Dec-19 19:43:08

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.

TeenPlusTwenties Mon 02-Dec-19 19:44:19

Charlie RTFT.

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

insufferablefury Mon 02-Dec-19 19:46:31

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 -9^2, and typing in (-9)^2 , you get different answers.

OP’s posts: |
CharlieWork Mon 02-Dec-19 19:49:46

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

CharlieWork Mon 02-Dec-19 19:51:46

Calculators are programmed to work using the BODMAS method

EggysMom Mon 02-Dec-19 19:55:13

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.

Join the discussion

To comment on this thread you need to create a Mumsnet account.

Join Mumsnet

Already have a Mumsnet account? Log in