-9^2 = -81

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.

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 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?

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.

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!!

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.

Frogsandsheep I have :)

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.

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

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 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. 😉

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

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’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!!

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.

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

It has been a brilliant thread, thanks OP.

Me too, Teen, thanks for the @ summons last night.

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

x2+5 Vs -x2+5
What do you get where x is +9 and when x is -9?
Its not clear from the original question which it is.

x=9 you get 86 Vs -76
x=-9 you get 86 Vs -76
The same for both so the answer is still 81 or -81 depending on whether its the a positive or negative 9. I appreciate that many expect negative numbers to be in brackets when shown on there own but we don't show the + for positive numbers. I can see it being -81 but I still would by default have said -9 squared on seeing that equation.

I'm sorry op. I wasn't calling you stupid I was equivalencing stupid equations. But I can see I was rude and I didn't mean to be.

I know that DadDadDad (we have the same professional qualification as well as similar degrees), but we also all know that that's not what the teacher was getting at.

Most people (including mathematicians) would glance and say 81 before thinking again I think.

I texted it to DH (PhD physics, works with more complicated mathematical models than I do) and he said 81, and only changed his mind when I pointed out x2+5 Vs -x2+5, and what putting a zero in front does etc.

4 x 6 = 24 / 2 = 12 cm^2 would I kick up a fuss? Top set, yes, bottom set I’d be bloody delighted they remembered to divide by 2.

...and pretty impressed that they'd shown the cm^2 as well, I bet!

(But incapable of rotating a picture it would appear - try again).

Very technically, the answer is -81.

The answer is -81, but it's just by the normal rules of algebraic notation. Nothing too technical about that. See picture.

(Credentials: Maths degree, ex-Maths teacher, use exactly this kind of calculation in my job)