Discussion in 'Mathematics' started by brookes, Oct 14, 2011.

2. ### DMNew commenter

Just kill me now.

3. ### mmmmmathsNew commenter

I take it this is a Friday night wind up!

4. ### AnonymousNew commenter

No - it came up. I can see the answer but the tutee wanted to know why. She substituted 4 into an equation and worked out the incorrect answer. She had a gut instinct why as she thought the curve would look wrong.
So - I don't know except to tell her to go through it step by step. We did not have the factorised equation - just the expanded version.
So yes - I am ignorant. Anyone care to explain?
Thanks

5. ### DMNew commenter

-20 + 6 does not equal - (20 + 6).
That aside addition does NOT take precedence over subtraction. They have equal status.
The order of operations is:
B
I
D or M
A or S

6. ### AnonymousNew commenter

Thankyou - I have revealed my ignorance. I didn't actually know that.

7. ### DMNew commenter

DM's law of the internet is that any question is a good question unless it is a BIDMAS question.

8. ### AnonymousNew commenter

Well a bit of research has revealed BEMA ( as used in New Zealand).
Dare I say I work mainly in primary schools but tutor upto GCSE? This has never come up before - and as Wikipedia says, it's one of the dangers of mnemonics.
That's two new maths things I've learnt this week.

9. ### DMNew commenter

Americans have the same problem with PEMDAS.

10. ### salsamaths

Yes, that's how I teach it too...

BUT

doing addition before subtraction would work provided that you did '16 + 6', first (the sign stays with the number it comes in front of, not after), and still get the correct answer?

Students struggle with this concept and so it's easier to say, once you get down to just add and subtract, work through from the first number to the last. (Do addition and subtraction in the order they appear in the calculation).

6 x 2 / 3 you could do 2 divided by 3 and then times 6.

11. ### salsamaths

Sorry that last message was in response to DM's

12. ### Woostarite

If works due to the commutativity (and associativity) of addition and by using a - b = a + (-b), where the minus on the left is a binary operator, and the minus on the right is the unary one.
But it's massively uninteresting, and as you say, no sane person would do anything except work left to right when dealing with just addition and subtraction.

13. ### 161051

I'm insane.

Often there's good reason to do it in a different order.

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

Extreme example.

14. ### AnonymousNew commenter

Just to say I knew the answer - but I normally work out quadratics by working left to right as a straight forward sum. She had put all the numbers in a row and applied BODMAS.
But - like I said, I have learnt something. Which is a good thing. Everyone should learn something new once in a while.

15. ### salsamaths

Also,
10 - 9 + 8 - 7 + 6 - 5 + 4 - 3 + 2 - 1.

Isn't maths fun?!!

16. ### rustybug

When I teach BIDMAS I always make a special point of reminding the class that addition and subtraction are the same operation (subtraction is addition of a negative number) and mult and div are the same operation, where division by 2 is multiplication by a half.
Then we have a bit of fun trying to say
BIMDAS
and
BIMDSA
and
BIDMSA
etc
(Another minute or so of fun to be had in working out how many correct versions there are of the mnemonic.)

17. ### AnonymousNew commenter

I think I was just having an off moment
But it did make me realise about BIDMAS and the actual rule - but if I was being a child who "sort of knew" BIDMAS, then you can see how mistakes might happen with
5^2 - 5 x 4 - 12 + 6
So error rectified - common sense intervened and I was very good in my reply compared to what I was going to write

19. ### 161051

I guess I misunderstood you both then. Sorry. Still, I know I'd have been upset if it was my thread and I think Robyn reacted very well.

On the matter of enduring things...you don't have to, you know and if people ignored annoying threads they wouldn't be very long or very annoying.

Just a bit more of my wisdom for you.

Now, I'll confess to being a bit of a hypocrite. I hate to see people teaching something when they lack the skills or knowledge or even just the confidence. But a person's teaching ability can change for the better so maybe I'll try not to be snotty to anyone and just help people who want to improve, when I have a clue what I'm talking about that is.

Oh, and well spotted DM.

20. ### AnonymousNew commenter

This never came up in any of my teaching and tutoring - we've done BODMAS, lots of complex calculations - then it just happened! Looking in my teaching books - doesn't mention the rules exactly. MyMaths - ignores it. But it's on BBC.
I do remember that equation recently. Makes you think.