-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?

My iPhone calculator has that separate +- just looked.

Agreed, Stealth, as shown in my Excel screenshot above. Whatever way a cell is formatted to appear, the entry of numbers into formulae still uses a minus sign for negative numbers. (So you type -9 and it appears as (9.00) if you have an accounting format).

My screenshot above shows that for Excel -92 = 81, but 0-92 = -81.

Agree it is a good thread. I like the part when actuaries get asked.

Marsha - I could email around 100 actuarial colleagues if you wanted more answers , but I wouldn't want to kill my inbox or spend the day refereeing endless discussions...

Love it sounds great

Too late, 1pm, lunchbreak over. Back to my spreadsheets...

Oh no didn’t mean do it. I like the sound of that world though.

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.

Upper primary here - and I am all over that kind of thing like a rash, so it is interesting that a secondary teacher would let it lie under some circumstances.

A key objective, even in early primary maths, is the understanding of the equals sign as a balance - and this example would show that a child doesn't have that basic understanding, which we would see as a significant issue.

4 x 6 = 24 / 2 = 12 cm^2 would I kick up a fuss?

I've always told DD2 not to do this, but she still does.
As she is now y10 I sometimes think I've bigger issues to worry about. The one I'm 'all over like a rash' is writing the blasted formulae down first, then putting in the numbers, then working it out!

It's not that they don't understand =, it's that they are rushing ahead to do the next bit and just consider the last thing they have written down. The = is history by that point as far as they're concerned.

I wonder if it's because they're used to using calculators? They do 4x6 and press equals, then press /2, then equals - and this transfers to written maths.

Almost certainly.

That's probably a key difference - in primary, calculators aren't used, so that sort of recording would imply a lack of understanding (of a key point that we teach throughout primary).

In secondary, the understanding may be there but the replication on paper of what is typed into a calculator results in this type of 'lazy' / inaccurate recording.

Ds’ maths teacher answered 81 although his exact words were ‘what’s -9 squared or -9 x -9’

cant it‘s not down to calculator use, I don’t think, it’s simply lazy recording when even doing basic calculations like the aforementioned area of a triangle.

You are rather unusual in being maths trained and primary, I don’t know if your other primary colleagues would have a mathematician’s natural visceral reaction to seeing that sort of monstrosity?

I had a massive argument with my A-level class the other day about an inequality expressed as 2>x>1. Unacceptable.

When you say it, it's difficult to hear the difference between
minus nine-squared
and
minus nine, squared

It's like me getting puzzled by a shampoo bottle saying "no buildup formula" wondering what "buildup formula" was and why they had removed it, when it's actually saying it has a formula to prevent buildup.

What's wrong with 2>x>1? I can see a problem with 2 < x < 1, but your example looks legal to me, even if it might be more conventional to write it 1 < x < 2.

1a might be technically correct but that doesn’t mean I’m not going to cry when they write it instead of a!

I suppose it depends on how you interpret the number line for a child.

As a student you are usually shown a number line when first learning maths and -9 represents a physical point within that number line and not a number plus an operation.

Following this line of thought -9 would not be perceived as (0-9) as this is not a point on the number line and so the equation would then become (-9)^2. I guess that’s where confusion can come into it

I would also like to add another thought that
-92 = -1 * (92)
Which also equals -81

I don’t know if your other primary colleagues would have a mathematician’s natural visceral reaction to seeing that sort of monstrosity?

I don't know. If I put it this way - that type of recording is rare in pupils in my year who have come up through my school (suggesting that we all care about it!) but it is very common in children arriving as 'in year admissions' into my year group (suggesting that as a school we are stricter on it than others are).

Are you maths lead, cant?

If we're now talking about writing things out clearly, my bugbear is I spend so much time trying to get DD2 to lay things out clearly (to help her get things right), and then on exam papers there's no room to do this so it positively encourages them to cut corners and make mistakes.
Plus in maths lessons they have squared books (I presume this is pretty standard) and again in an exam they are suddenly expected to do stuff on blank paper. No other subject expects this.

@StealthPolarBear, thank you for being kind.

And thank you everyone, although a lot of things went over my head tbh, it was great that I got an answer and also created great discussion among maths specialists. It was very interesting to read, and good to know that there are so many passionate teachers and maths specialist on MN who are willing to help us parents.

@DadDadDad,, substituting 9 with x really made sense to me. Great explanation. Thank you.

Ha, just saw your post @insufferablefury as I was helping my DD with her maths homework, and she was telling me "you explained that so much better than my maths teacher!" I'm on a roll.

For balance, I should point out the many times my children have told me that my maths explanations make no sense and I obviously don't have a clue what I am talking about!