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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.]

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WooohVerOfPaper · 31/10/2020 15:29

I “think” we understand now. I think the problem was being taught a set of rules rather than being taught the concept, the why. When the rules changed ( different teacher) panic and anxiety set in.
We have now got homework to do!
Thank you all!

Lougle · 31/10/2020 10:09

6 - 2 + 4 x 5 / 2 + 9

Brackets first: There are none.
Order second: There are none.
Multiplication or Division third: There are both.

4 X 5 / 2. You can choose whether to do:
(4x5)÷2 which is 20÷2 = 10
Or
4 x (5÷2) which is 4x2.5 = 10

Once you've done that, you can rewrite your sum: 6 - 2 + 10 + 9

Now you only have Addition and Subtraction to do. As it doesn't matter which order, you can either add all the positive numbers together (25) and subtract 2, making 23, or do the sum in order 6 - 2 (4) + 10 (14) + 9 = 23.

TeenPlusTwenties · 31/10/2020 08:19

6 - 2 + 4 x 5 / 2 + 9

First multiplication & division

4 x 5 / 2 you can do as either 4x5=20 divided by 2 = 10 or 5/2 = 2.5 then 4 times that which miraculously still = 10

So now we have 6 - 2 + 10 + 9

you can do that in any order you like eg
6 add 10 add 9 makes 25, subtract 2 makes 23

or 6 subtract 2 makes 4 add 10 makes 14 add 9 makes 23

or negative 2 add 10 makes 8, add 6 makes 14 add 9 makes 23

BUT you must make sure that negative sign sticks tightly to the 2
6 - 2 + 10 + 9 What you must not do is say 2 add 10 add 9 makes 21, so 6 subtract 21 is negative 15.

The - sign means negative 2, it doesn't mean subtract everything that comes to the right of it until you get to the end.
ie it is ABSOLUTELY NOT 6 - (2+10+9)

Timothyjohn · 31/10/2020 00:50

Still don’t understand if the multiplication and division takes order precedence over addition and subtraction in a sum like the one below

6 - 2 + 4 x 5 / 2 + 9

Would that mean that according to the bidmas/bodmas rules (two names for same thing ..... it’s super easy kids) would mean

Order of events for sum
6 - 2 + 4 x 5 / 2 + 9

4 x 5 first
Then divide that by 2
Then do the first bit 6 - 2
Then finally + 9

Please can somebody really spell it out this system like I’m a kid as it’s not going in Blush

CaraDuneRedux · 30/10/2020 23:22

It’s better that the mathematical principles are taught without over reliance on sticking rigidly to the acronym.

True...

Though I'm starting to have traumatic flashbacks to my first year undergrad mathematical logic course and Polish notation and operator scope Grin.

en.wikipedia.org/wiki/Polish_notation

Don't think I ever quite understood it. I mean, I sort of got what it was about conceptually, but actually using it in anger? Hell, no! Confused

PurpleDaisies · 30/10/2020 23:13

@nicerbeing

The whole point of BODMAS/BIDMAS is that you do it in that order.
But you don’t have to do it strictly in that order. It’s better that the mathematical principles are taught without over reliance on sticking rigidly to the acronym.
CaraDuneRedux · 30/10/2020 23:12

5 x 5 - 3 / 2 + 2

The key thing is you have no brackets, so multiplication and/or division take precedence over the addition/subtraction. So start with the stuff that needs multiplied or divided.

That will give you constituent bits for your eventual sum as follows
5 X 5
3/2
2

giving you 25, 1.5, 2 which you then put together thus: 25 - 1.5 + 2

25.5

(or if you prefer to think of it Purple's way, you have
25, -1.5, 2 which have to be added up).

nicerbeing · 30/10/2020 23:10

The whole point of BODMAS/BIDMAS is that you do it in that order.

PurpleDaisies · 30/10/2020 23:08

(5 x 5) - (3 / 2) + 2

25-1.5+2 =25.5

dementedpixie · 30/10/2020 23:05

1.Brackets
2.Indices/Order
3.Dividing/ Multiplication (of equal importance)
4.Adding/Subtraction (of equal importance)

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 ?

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.

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.

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.

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.
PurpleDaisies · 30/10/2020 19:25

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

Sarjest · 30/10/2020 19:24

Yes, it is Y7. Good luck!

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.

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

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?

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.)

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

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...

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.

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