Active discussions

Please or to access all these features

Talk
Secondary education

Connect with other parents whose children are starting secondary school on this forum.

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:
Thread gallery
7
myduckiscooked · 02/12/2019 19:23

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.

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

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 posts:
TryingAndFailing39 · 02/12/2019 19:10

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

hiddenmnetter · 02/12/2019 19:07

I see I've cross postedXmas Blush

hiddenmnetter · 02/12/2019 18:49

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)

Original poster
insufferablefury · 02/12/2019 18:48

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 posts:
Original poster
insufferablefury · 02/12/2019 18:45

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

OP posts:
Original poster
insufferablefury · 02/12/2019 18:42

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

OP posts:
StealthPolarBear · 02/12/2019 18:42

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.

Xiaoxiong · 02/12/2019 18:42

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

ItsGoingTibiaK · 02/12/2019 18:41

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

Original poster
insufferablefury · 02/12/2019 18:40

StealthPolarBear, I know we normally don't need to use brackets. The question we got stuck was -92, so I used brackets to show my dc the difference in -(92) 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 -92. to get 81. Seems to be very confusing when teacher says the answer for -92 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 posts:
Xiaoxiong · 02/12/2019 18:40

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.

-3^2=-9
(-3)^2=9

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.

Teachermaths · 02/12/2019 18:33

It depends on the question....

Is it substituting - 9 into an expression?

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

GuyFawkesDay · 02/12/2019 18:31

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

StealthPolarBear · 02/12/2019 18:29

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.

StealthPolarBear · 02/12/2019 18:28

(-9)^2 is 81