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Bidmas again! 9-9+9

93 replies

redhat · 20/01/2016 17:40

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......Blush

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redhat · 20/01/2016 17:40

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

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soapybox · 20/01/2016 17:42

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

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redhat · 20/01/2016 17:43

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

aaaghhh!

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merrymouse · 20/01/2016 17:44

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

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noblegiraffe · 20/01/2016 17:44

Left to right so = 9

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icklekid · 20/01/2016 17:44

No its 9. 18-9 not 9-18

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soapybox · 20/01/2016 17:45

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

Remember the - sign attaches to the number.

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slicedfinger · 20/01/2016 17:46

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

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redhat · 20/01/2016 17:46

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

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merrymouse · 20/01/2016 17:46

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.

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IfItsGoodEnough4ShirleyBassey · 20/01/2016 17:47

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

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redhat · 20/01/2016 17:48

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

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merrymouse · 20/01/2016 17:48

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

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merrymouse · 20/01/2016 17:49

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

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redhat · 20/01/2016 17:49

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

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redhat · 20/01/2016 17:51

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!

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merrymouse · 20/01/2016 17:52

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.

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vvviola · 20/01/2016 17:53

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

+9
-9
+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

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merrymouse · 20/01/2016 17:54

What maths topic is he studying?

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WhiskyTangoFoxtrot · 20/01/2016 17:56

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

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user789653241 · 20/01/2016 17:58

BIDMAS Blaster/ BIDMAS blaster Lite on

www.mangahigh.com/en-gb/

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

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redhat · 20/01/2016 18:11

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

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Hulababy · 20/01/2016 18:15

www.mathsisfun.com/operation-order-bodmas.html

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.

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user789653241 · 20/01/2016 18:16

www.khanacademy.org/math/pre-algebra/order-of-operations/order_of_operations/v/introduction-to-order-of-operations

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

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Hulababy · 20/01/2016 18:17

BODMAS/BIDMAS showing the equal operations

Bidmas again! 9-9+9
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