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Bidmas again! 9-9+9

(94 Posts)
redhat Wed 20-Jan-16 17:40:21

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


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......blush

redhat Wed 20-Jan-16 17:40:40

Oh FFS I didn't mean 18 I meant -9!!

soapybox Wed 20-Jan-16 17:42:29

It doesn't matter which order you do this in, the answer is always going to be 9!

redhat Wed 20-Jan-16 17:43:16

No if you do the addition first you get 9+9 as 18 and then you have 9-18 surely? i.e. -9


merrymouse Wed 20-Jan-16 17:44:18

I agree, Bidmas doesn't affect that sum. The numbers could be in any order and the result would still be 9.

noblegiraffe Wed 20-Jan-16 17:44:39

Left to right so = 9

icklekid Wed 20-Jan-16 17:44:40

No its 9. 18-9 not 9-18

soapybox Wed 20-Jan-16 17:45:35

No the addition is -9 + 9 = 0 then you add the first 9 to get 9.

Remember the - sign attaches to the number.

slicedfinger Wed 20-Jan-16 17:46:11

If you put brackets round the 9+9, then you'd do it first. No need to with this one. Stop panicking!

redhat Wed 20-Jan-16 17:46:23

Right so is the rule that you do addition and subtraction last but they rank equally so you go left to right?

So 9-9 = 0 + 9 = 9 ??

Im confused by the 18 -9

merrymouse Wed 20-Jan-16 17:46:50

You would have 9+9=18 then 18-9=9.

Basically you have a '-9' and two '+9's. Which ever way you add them together the answer will always be 9.

IfItsGoodEnough4ShirleyBassey Wed 20-Jan-16 17:47:18

BIDMAS is really BI (D/M) (A/S). Addition and subtraction have equal precedence and are carried out left to right.

redhat Wed 20-Jan-16 17:48:12

Ok now I am completely confused (and DS is almost in tears). Please take me through this gently and slowly

merrymouse Wed 20-Jan-16 17:48:24

If you went right to left the sum would be +9-9+9=9.

merrymouse Wed 20-Jan-16 17:49:03

Ignore Bidmas for this sum, just add up the numbers.

redhat Wed 20-Jan-16 17:49:23

because they rank equally and so we go left to right?

redhat Wed 20-Jan-16 17:51:53

So I'm telling DS that division and multiplication are equally important and so go left to right. Addition and subtraction are also equally important and go left to right.

I thought Id done well knowing the order but obviously forgot that essential piece of information!

merrymouse Wed 20-Jan-16 17:52:47

You can go right to left or left to right when you are adding a series of numbers.

You have a 9, a 9 and then a -9.

It's just like adding up 2, 5 and 6. Doesn't matter how you add them you will always get 13.

vvviola Wed 20-Jan-16 17:53:13

Put them on top of each other OP, to see how it can only be 9.


One -9 and one +9 cancel each other out so that answer is +9

But essentially just do it left to right and it will be fine.

Even if you did it with the last 2 numbers first it would be the same. Look.

9 - 9 + 9

Taking the last 2 numbers
-9 + 9 = 0
Then add the 1st 9 = 9

merrymouse Wed 20-Jan-16 17:54:13

What maths topic is he studying?

WhiskyTangoFoxtrot Wed 20-Jan-16 17:56:04

You can do them in any order.

The first number is a positive number, but by convention you don't write the '+' but putting it in might help you. So any order will give the same result (which is what is meant by addition and subtraction having equal precedence).

So for this sum, you could write is three ways and they all give the same answer:
+9 -9 +9 = 9
+9 +9 -9 = 9
-9 +9 +9 = 9

irvine101 Wed 20-Jan-16 17:58:49

BIDMAS Blaster/ BIDMAS blaster Lite on

is really good. (you need to register but it's free.)

redhat Wed 20-Jan-16 18:11:07

He's studying Order of Operations but I'm not sure they've had a great deal of explanation from the teacher on it yet. I think I confused him when telling him the order because he then thought he had to do the addition before the subtraction.

That makes sense though thanks all <wanders off praying that DS never does A Level maths!>

Hulababy Wed 20-Jan-16 18:15:53

Explains it ...

A/S are equal, go left to right

So, 9-9 = 0 +9 = 9

Similarly M/D are equal and worked left to right too.

irvine101 Wed 20-Jan-16 18:16:53

This is American, so seems to be called PEDMAS, but it's good explanation.

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