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Original poster

BIDMAS BODMAS

35 replies

MoverOfPaper · 30/10/2020 18:25

Help!

Argument ragging in my house over Y7 maths homework...

Which is correct?

1.Brackets
2.Indices/Order
3.Dividing
4.Multiplication
5.Adding
6.Subtraction

Or

1.Brackets
2.Indices/order
3.Whichever is furthest right of division or multiplication then work from right to left
4.Whichever is furthest right of addition and subtraction and work from right to left?

DD has been taught both ways at various times (she thinks) by different teachers or teaching assistants.

Thank you anyone. I would call NobleGiraffe but hopefully she is up to her neck in gin and not thinking about school.

If you could also provide an example of how to do it I would be so grateful

the question is;

use operations (+ , - , x , /, () to make 12345*6=44 a true statement.

my notes [/ means division but we can't work out how to get that symbol.]

OP posts:
Original poster
MoverOfPaper · 30/10/2020 18:28

argh...which is furthest LEFT and then working left to right*.

OP posts:
hercatsanddogs · 30/10/2020 18:28

The first one

IsolaPribby · 30/10/2020 18:31

((1+2+3+4)*5)-6

I think that works?

woodhill · 30/10/2020 18:33

First one

PurpleDaisies · 30/10/2020 18:36

There is no mathematical rule about having to go left to right for multiplication/division or addition/subtraction. That is often taught to help students not to miss out parts of their calculation but you get the same results either way. Teaching left to right as a strict rule sometimes reflects a lack of understanding about the orders of operations.

It is more helpful to think of it as
Brackets
Indices
Multiplication and division
Addition and subtraction

WooohVerOfPaper · 30/10/2020 18:48

Eek. Now I’m confused.

BBC Bitesize mentions the left to right method.

Could you do

Brackets
Indicie
Division
Multiplication
Addition
Subtraction

Or

Brackets
Indicies
Multiplication
Division
Subtraction
Addition

For the same problem and both methods be correct?

Meredusoleil · 30/10/2020 18:51

5(1+2+3+4)-6

I would say the correct order is as pp said:

  1. Brackets
  2. Indices/Order
  3. Division and Multiplication
  4. Addition and Subtraction
TeenPlusTwenties · 30/10/2020 18:51

The second one is 'more' correct as Purple says.

Isola's answer works for the question. You can always put brackets in to be extra clear on meaning (helps in computer science, but obviously not so much in showing BODMAS).

PurpleDaisies · 30/10/2020 18:53

You’re right @WooohVerOfPaper

It doesn’t matter whether you divide then multiply or vice verse, and the same for addition and subtraction. Sometimes deliberated choosing to divide first rather than multiply makes the problem easier to solve rather than just blindly following the “BODMAS” acronym.

Bobkitten · 30/10/2020 18:55

Wouldn’t the below work - keeps the numbers in order and also avoids double brackets which may be a bit much for some year 7s?
(1x2) + (3x4) + (5x6) = 44

LittleMissnotLittleMrs · 30/10/2020 18:55

B I {D,M} {A,S}

D M are inverse operations so can be rewritten as the other. Likewise A S. So x0.5 is the same as /2.

Hence L to R once priorities are sorted

TeenPlusTwenties · 30/10/2020 18:55

If you have 10-8+7 you have to get the answer 9.
What you don't want to do is calculate it as if it said 10-(8+7) by doing 8+7 before subtracting.
As long as you realise that you are probably OK.

Now trying to think of an example for the * / issue.

Varjakpaw · 30/10/2020 18:57

We were taught BIMDAS at school (1970s). Our maths teacher spent a whole lesson illustrating why BIDMAS was wrong. He was a brilliant teacher and made everyone love maths. Doesn’t help but it did remind me of him...

PurpleDaisies · 30/10/2020 19:02

For your example tween, my students have been battered into knowing that the - sign cannot be separated from the number after it.

So... 10 + (-8) + 7

7 + -8 = -1

10 + (-1) = 9

CaraDuneRedux · 30/10/2020 19:03

Purple is right. Maybe a worked eg will help.

First example
6 5/ 2+ 3 = 6 / 2 5 + 3 = 18

Doesn't matter what order you do the multiplication and division in.

Second example
But (4 + 5) / 3 = 3
Brackets tell you to do the addition first

Versus
4 + 5/3 = 5 2/3
In the absence of brackets you do the division first

Give you different answers.

Third example
5 + 1 - 3 = 5 - 3 + 1 = 3
Doesn't matter what order you do the addition and subtraction in.

(But note: because it doesn't matter, this means you won't get it wrong by arbtrarily telling children BIDMAS such that they always do division before multiplication and addition before subtraction. You could equally well have taught them BIMDSA, but conveniently it doesn't matter.)

WooohVerOfPaper · 30/10/2020 19:18

Thanks. I’ll show this to DD-tomorrow. Can’t take anymore tonight.
No, it doesn’t make sense to me. I scrapped a C at the early GCSE where we used calculators. DH is better at maths than me but less qualified!
Is this usual Y7 work?

bravefox · 30/10/2020 19:23

Fwiw America they say PEMDAS:

Parentheses (brackets)
Exponent (powers)
Multiply
Divide
Add
Subtract

-just posting to reinforce the point mult/divide can be done either way around. Same for add/subtract

TeenPlusTwenties · 30/10/2020 19:24

Yes it is usual y7 work.
They need to recap BODMAS (done at primary) before they can do algebra.
They need to understand that 3y^2 where y=4 equals 48 and not 144.

Sarjest · 30/10/2020 19:24

Yes, it is Y7. Good luck!

PurpleDaisies · 30/10/2020 19:25

This is normal for year 7. I’ve done orders of operations with year 6.

CaraDuneRedux · 30/10/2020 19:32

@PurpleDaisies

For your example tween, my students have been battered into knowing that the - sign cannot be separated from the number after it.

So... 10 + (-8) + 7

7 + -8 = -1

10 + (-1) = 9

It's a good way of making sure the students always get it right, even the ones for whom maths doesn't come naturally, but it does obscure the fact that addition and subtraction are mathematically inverse operations (as are multiplication and division). Though I guess any kids who're going to get into that level of mathematical abstraction probably won't be phased by this in the slightest.
Lougle · 30/10/2020 19:41

@TeenPlusTwenties

If you have 10-8+7 you have to get the answer 9. What you don't want to do is calculate it as if it said 10-(8+7) by doing 8+7 before subtracting. As long as you realise that you are probably OK.

Now trying to think of an example for the * / issue.

It doesn't matter, though, whether they do

10 - 8 (2) +7 = 9
or
10+7 (17) - 8 = 9

Because the - sign is attached firmly to the 8.

WooohVerOfPaper · 30/10/2020 20:39

Oh gosh. Usual for year 7 then? I expected a lot of going over old ground, over learning, working out where the kids where, post the March-July shutdown.
She was taught BIDMAS in primary but she didn’t understand it then either.

IdblowJonSnow · 30/10/2020 21:24

Eek! My year 6 dd has just done this. She loves maths and is great at it. Me, not so much! Either we never did this at school or I've blocked it out!
Can't think of a situation where I personally might be required to use it.

Timothyjohn · 30/10/2020 22:59

@PurpleDaisies

For your example tween, my students have been battered into knowing that the - sign cannot be separated from the number after it.

So... 10 + (-8) + 7

7 + -8 = -1

10 + (-1) = 9

I understood that the brackets part of the sum have to be dealt with first ? If so I don’t understand the examples above ?

Also can somebody please clarify the order that you have to follow to solve the following sum?

5 x 5 - 3 / 2 + 2

To solve the above would you have to tackle the multiplication and division aspects before you do the subtraction and addition aspects ?