Bidmas again! 9-9+9

[redhat]

I can't believe I am failing to assist my child with year 5 homework!

9-9+9

Is this 18 because you do the addition before the subtraction? (I thought it was this but think I'm wrong)

Or is it 9 because the addition and subtraction rank equally and so you go left to right?

I knew I should have paid more attention in school......

[KatharinaRosalie]

I teach Maths and can assure you BIDMAs is the order

Well, that explains all those BIDMAS threads on MN and everywhere else, where we have 60 pages of people arguing over order of operations - they have been taught wrong.

[HarveySchlumpfenburger]

That's the explanation I would always go with to children noble. It's one less thing to have to remember or understand. If you don't have to manipulate the numbers and work out what goes with what sign, why bother?

With older or more able students who have a very firm understanding of subtraction being the same as adding a minus number and who can experiment with that a bit it might be an interesting investigation/discussion about how and why it works.

For most, it's probably an added layer of confusion.

[user789653241]

I teach Maths

I feel sorry for your students... I am really shocked.

[Galena]

mumneeds I worry that you are teaching maths with such a ropy understanding of basic mathematical rules.

9 - 9 means 9 + -9

Addition is commutative, so a+b = b+a.

Therefore, 9 + -9 + 9 = 9 + 9 + -9 or 9+9-9 = 18-9 which is 9.

6-7+3 = 6 + -7 + 3 so, using the commutativity of addition, this means you can add 6 and 3 before subtracting 7, giving 2. Otherwise you'd add the 7 and 3 first before subtracting from 6 which would give -4.

One way to think about it is that positive numbers/addition are going up in a lift and negative numbers/subtraction are going down in it.

9-9+9 therefore means 'go 9 floors up, 9 floors down and then 9 floors up.' If you do that, you end up 9 floors higher than you started.

6-7+3 means 'up 6, down 7, up 3' not 'up 6, and down seven add three which is 10'

If you really STILL think you are right, PLEASE ask your head for some maths training. I am an advocate for maths, having a maths degree and having served many years in the Mathematical Association, and this sort of teaching (i.e. teaching where the teacher doesn't understand and is teaching incorrectly) can cause years of issues as teachers further down the line try to undo the misconceptions taught by one rogue teacher.

[HarveySchlumpfenburger]

The sticky at the top of this board is for a maths Q&A.

Perhaps we need to ask the question!

[user789653241]

I agree with Galena. I learned maths in 2 different countries. They both taught me add/sub and times/div is considered equal in order of operations. So, you are really wrong. Please think again before spreading more damage to your pupils.

[Deux]

Can't you also put the sum into a proper calculator, exactly as it is written, to check the right answer?

[HarveySchlumpfenburger]

You can. Unsurprisingly, it gives the answer as 9, not -9.

[Deux]

Exactly. Surely a maths teacher know this?

[Hulababy]

Mumneedswine- what age are you teaching?

I also teach - but would teach as and md as equal. As a and s as essentially the same operation (just inverse).

[DropYourSword]

Please tell me mumsneedwine is actually kidding us on about teaching maths!

[lougle]

4x12/6x5+5-3

Multiply or divide order doesn't matter:
4×12/6 can be (4×12)/6=48/6=8; or 4×(12/6)=4×2=8
Either way the 8 is then ×5: 8x5=40.

40+5-3 again the order between add and subtract is irrelevant: 40+5-3=45-3=42 or 40-3+5=37+5=42.

[HarveySchlumpfenburger]

Going back to read mumsneedwine's post again. She did concede that the answer was 9 because it should be -9+9 = 0

And the issue with the example she gave isn't that her working out is wrong, its that she's misread her own question.

4x12/6x5+5-3

You can work out the 12/6 first or go from left to right, it doesn't make any difference to the answer. The problem was she transcribed the answer to 12/6 as 3. Then when she went to type the next line she mistyped the 4 as a 5 which gave her 5x3x5=75 rather than 4x2x5=40.

It's still wrong to insist that subtraction must be done last though. It can be done last but you have to be really careful about what you are adding.

[user789653241]

If she can't calculate 12/6, she shouldn't be teaching maths in first place. I can understand miscalculation can happen, but I would like them to admit that she made a mistake, and correct it. Or if she really think she is right, she should show us why.She just disappeared from this thread.

I can see the scenario that some child saying, "Mrs mumsneedwine, I think the answer is 42", and she saying "No, it's not. I am the teacher. I am right, because..." and the child getting really confused. How can it even be possible to happen? The teacher who doesn't know the basic teaching....I just hope she doesn't teach at my son's school.

[HarveySchlumpfenburger]

She calculated it correctly. When she typed the next line she changed from 2 to a 3 which screwed the rest of the calculation. It's the sort of mistake that used to cause my maths teacher to write 'careless error' all over my maths book.

Her insistence that division and multiplication or addition and subtraction don't have the same rank and must be done in exact DMAS order rather than left to right is still wrong. It can be done both ways.

[user789653241]

Yes that's exactly what I mean. As a maths teacher, I expect her to check what she has posted and saw the mistake, and correct it. I haven't got a clue why she mistyped all those numbers wrong, but she hasn't got a decency to come back and correct it, which make me think she is a type of person who never admits her mistakes. So, I really worry about children she teaches. It's no shame to admit your mistake in site like this, nobody knows you. And she really should be grateful that she found out she was wrong, not in front of her pupils or other teachers.

[PurpleDaisies]

I'm not worried at all that the she got a sum wrong through mistyping (having done exactly the same thing about three posts later!).

The total refusal to accept that division/multiplication and addition/subtraction can be done in either order in the face of plenty of academic websites showing that they can (not to mention other posters) was more concerning.

[user789653241]

No, purple, I'm not worried about her mistyping either. What I'm worried about is exactly what you say. Total refusal to the fact. I just wish she reads this thread again and think again.