-9^2 = -81

[insufferablefury]

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?

It has been a brilliant thread, thanks OP.

It’s not ‘very technically -81’, it is -81.

@mummmy2017 did your daughter saying -92 = 81 or -9x-9 = 81 as it’s not technically the same thing. If an a level teacher said -92 = 81 written exactly as it is then I’d be a bit

And I would make my year 7s put the brackets in. When we do indices in the lower school I always do
(-6)2 and -62 in my examples to indicate the difference and the importance of brackets. In algebra it’s really important to know the difference.

I’ve noticed how none of the cocky pps from the first couple of pages who were quite rudely wrong have come back to the thread!!

I am feeling relieved relieved here that the consensus has shifted towards the OP. My mere Engineering degree has stood up to scrutiny. I’ll have to let Howard Wolowitz know.

I am really surprised that this discussion has led to 6 pages, so far, about what -9^2 is though.

I have been known to hand over a calculator and ask them to find the square root of -81.

I love a good maths thread. Cheers me up no end.

Me too. I just wish MNHQ would implement a proper mathematical typesetting system so we can really treat them seriously. And a CAD module for parking diagrams. 😉

Same for my degree in applied mathematics and statistics

It's just a rule to learn. Brackets first (if present), then exponents, then negation. All the people earlier in the thread who were arguing for +81 just don't know the rule (or are applying it backwards), thinking that a negative in front of a number is somehow stuck to it like superglue to make a negative number, regardless of any exponent.

I know Tibia!! When my post upthread went all wonky with the italics I actually considered getting out Latex and doing some screenshots...

Daddaddad I’m loving your choice for explanations. I teach Engineers, who are often very visual people, as much as I humanly can using graphical supports to back up the concepts. Quadratics, Integration, Differentiation, Linear Equations, the list is endless but even then I would not have thought of handling this question graphically. Well played there.

Demos is my favourite teaching tool.

Frogsandsheep I have :)

Thanks, myduck - I'm definitely a visual learner and I love finding a picture that makes things clearer, as you'll see from this thread at the weekend for maths-lovers who missed it: www.mumsnet.com/Talk/_chat/3756754-maths-brains-help-me-please

Now I'm at work, and ( @MindyStClaire please note ) planning to ask half a dozen actuaries the question of this thread. Will report back when I get a coffee break.

I asked 4 upper 6th boys last night. Two of them got -81 straight away. One of them asked whether there should be brackets around the -9 or not. One of them bashfully said knew it was wrong but asked why it wasn't +81 because the square of a negative is a positive. I drew your graphs Dad!!

Haha Dad you are officially in my visual world, brilliant. Engineering is great for it, we have a world of applications to help us with our visualisation tools.

The answer is definitely -81, I just don't think the teacher asked the question she intended to. And I think it's more understandable in this instance than the so many horrific BOMDAS threads I've seen on here that show teachers are teaching maths without understanding the basics.

Now, 8 ÷ 4 x 2 anyone?

DD asked her maths teacher
He said the answer wanted will be 81.
He marks exams for year 11.
If anything else is wanted brackets will be used

The problem here is typesetting I'm sure. Back when all this were just fields we'd be given a negative number with the - in superscript so you could tell it wasn't a subtraction symbol. I still write minus numbers in that way now.

But the minus sign in a negative number is a subtraction symbol.

Three actuaries said -81, one (my manager ) said +81. The trainee who has a Physics PhD offered explanations for both answers but went with -81.

By the way, I am aware that Excel doesn't help. It treats a unitary minus sign differently from an operational minus sign - see attached.

DD asked her maths teacher
He said the answer wanted will be 81.
He marks exams for year 11.
If anything else is wanted brackets will be used

Link the teacher to this thread

Excel is different, it uses brackets to demonstrate negative numbers.

But if the exam board want the answer of 81, then teachers are teaching the children to do basic maths, the EU can board are adding brackets to leave the children in no doubt of how to do the sum.
To confuse school age children at this state in their education will simply lead to them losing marks.

But if the exam board want the answer of 81

If an exam board wanted an answer of 81 the question would be asked in a different way to the OP. I can 100% guarantee if the question asked the way it was framed in the OP was answered as 81 by an examiner there would be an outcry. The answer to the OPs question as it is asked in her OP is -81.

"
Excel is different, it uses brackets to demonstrate negative numbers."
It might do that if you pick a specific format setting but it isn't a universal truth. I don't have it set that way, excel shows me negatives with a minus sign.

I just found my old calculator under a pile of rubble, it has a separate +|- button on it to differentiate between negative numbers and the subtraction operation. So much clearer for students back in the day.