WouldBe
Dislicksick
The order in which you work sums out
B = brackets
O = cant remember
D = division
M = multiplication
A = addition
S = subtraction
Maths homework anyone?
- Thread starter moomoo
- Start date
The order in which you work sums out
B = brackets
O = cant remember
D = division
M = multiplication
A = addition
S = subtraction
softybabe
one merlot then two....
B=bracket first then..
O= of
D=division
M=Multiplication
A=Addition
S=Subtraction
you have to do the solution in that order...so things in brackets get done first...then...
WouldBe
Dislicksick
Why though?
*Cries*
Because you'll get the wrong answer if you don't.
From your opening post.
10 + 10 / 10 + 10 = 1
If you just do that calculation in order you get 10+10 = 20
20/10 = 2
2+10 =12
which is a different answer to the one given.
If you just use BODMAS on the original example you do the 10/10 to start which gives 1 then your left with 10+1+10 which gives 21 and a third answer.
goldenecitrone
post tenebras lux
I'm having a few problems aswell. Can someone tell me the derivative of
4x squared (3 - 2x) to the power one third.
And also express sin 5A in terms of powers of sin A.
Cheers.
4x squared (3 - 2x) to the power one third.
And also express sin 5A in terms of powers of sin A.
Cheers.
Say you had an equation like this:
5 x ( 5 + 10 ) / 3 - 2
Then you'd do brackets first: (5 + 10) = 15 giving you 5 x 15 / 3 - 2
Then the divide: 15 / 3 = 5 giving you 5 x 5 - 2
Then the multiply: 5 x 5 = 25 giving you 25 - 2
Then the subtract: 25 - 2 = 23
If you have a power like 2^2 then you'd do that after the bracket, but I'd guess they're something you shouldn't have to worry about for a while yet!
5 x ( 5 + 10 ) / 3 - 2
Then you'd do brackets first: (5 + 10) = 15 giving you 5 x 15 / 3 - 2
Then the divide: 15 / 3 = 5 giving you 5 x 5 - 2
Then the multiply: 5 x 5 = 25 giving you 25 - 2
Then the subtract: 25 - 2 = 23
If you have a power like 2^2 then you'd do that after the bracket, but I'd guess they're something you shouldn't have to worry about for a while yet!
O = Ordinals (i.e. squared, cubed, etc)
So you always do it in that order, never mind what order they are in in the sum?
Yes. That's what the point of brackets is. You need them to show what needs to be worked out first. You might think what is the point but if you think about doing a sum that involves real things it makes more sense.
FridgeMagnet
Administrator
You should just get your kid to do it to be honest, it would be easier.
softybabe
one merlot then two....
Yeah, basically you're trying to make the equation as simple as possible before you solve it.
goldenecitrone: I believe you'll need to use differentiation by the product rule and the chain rule (been a good few years so I can't guarantee this is correct):
If y = (4*x^2)*(3-2*x)^(1/3) then set u = 4*x^2 and v = (3 - 2*x)^1/3
Then differentiate to give dy/dx = v*(du/dx) + u*(dv/dx)
To get dv/dx you'll need to use the chain rule as well: set w = 3-2*x, so v = w^(1/3) and then solve dv/dx = (dv/dw) * (dw/dx)
goldenecitrone: I believe you'll need to use differentiation by the product rule and the chain rule (been a good few years so I can't guarantee this is correct):
If y = (4*x^2)*(3-2*x)^(1/3) then set u = 4*x^2 and v = (3 - 2*x)^1/3
Then differentiate to give dy/dx = v*(du/dx) + u*(dv/dx)
To get dv/dx you'll need to use the chain rule as well: set w = 3-2*x, so v = w^(1/3) and then solve dv/dx = (dv/dw) * (dw/dx)
spanglechick
High Empress of Dressing Up
I'm getting grief from the English teaching massive now.
what did you call me??????
He was crying over it though.
I know the feeling. The horror of homework on a Sunday night. Thought it was over when I stopped being a student. Wrong wrong wrong.
FridgeMagnet
Administrator
Well, I was being a bit cheeky I admit, but the thing is, from forums you'll be getting half a dozen different conceptual explanations. I mean, I was looking through the above, and if asked to explain, I would do it a slightly different but similar-enough-to-cause-confusion way. And then he goes in to school and the teacher tells him _another_ different way! Not getting the answer right should be a signal to the teacher that he doesn't understand and needs to be shown how it works, getting it right isn't the goal in itself. If you see what I mean.
The maths degree was fun, the stuff at school not so much 
yep, chain rule for the first one
for the second it's a bit messy. You have to break it down in steps using the rules
sin (A+B) = sin A cos B + cos A sin B
cos (A+B) = cos A cos B - sin A cos B
cos²A + sin²A = 1 of course
I just worked it out as sin (5A) = 16 (sin A)^5 - 20 (sin A)^3 +5 sin A but could very easily have made a mistake
I do understand all that but when he starts crying and I can't help and his sister is doing her coursework and refuses to help him and the lad up the road is out it sends me into a bit of a panic.
I'm hoping there is going to be a parents evening soon anyway as he is really struggling with maths so I want to speak to his teacher about it.
Jonti
what the dormouse said
I think you're crying because you're trying to understand the Why?Why though?
*Cries*
But there's nothing to understand; it's just a convention, like driving on the left hand side of the road.
There has to be some sort of rule or convention about the order one does the component sums, 'cos you get different answers by doing them in different order.
The alternative to having some kind of convention (and the world uses the BODAS convention) would be to use lots of brackets to take away the ambiguity (brackets mean to do the sums deepest inside the brackets first) and dictate the order of doing the component sums that way.
Thing is, all the brackets would clutter the page and make it look untidy and distract one's eyes from the actual numbers.
That's why.
Thank you Jonti. I think I understand but it still seems odd to me to jump around the sum randomly rather than do it in the order it is presented iyswim. But I'll sit down with the boy tomorrow and explain all that to him.
Jonti
what the dormouse said
Oh, I do see what you mean alright. And yes, that would be another possible convention.
As a matter of interest the convention you suggest (just start on the left and work through the sums) is actually used by some computer programming languages.
Hmm, you've got me thinking now. I wonder why that simpler convention not used for paper and pencil workings. Hmmm.
I suspect because it is actually easier in practice to see the effect of a change in the expression (say, add a new term or two) with BODAS.
As a matter of interest the convention you suggest (just start on the left and work through the sums) is actually used by some computer programming languages.
Hmm, you've got me thinking now. I wonder why that simpler convention not used for paper and pencil workings. Hmmm.
I suspect because it is actually easier in practice to see the effect of a change in the expression (say, add a new term or two) with BODAS.
Thank you Jonti. I think I understand but it still seems odd to me to jump around the sum randomly rather than do it in the order it is presented iyswim. But I'll sit down with the boy tomorrow and explain all that to him.
Imagine a shopping list where you have washing powder, 3 pints of milk, a kitkat, 5 samosas and 4 packets of loo roll
then your till receipt would effectively say
total = 2.79 + 3*0.40 + 0.45 + 5*0.44 + 4*1.79
Which is easy to understand and write. It makes sense. But if you were going to calculate sums like that in the order presented, you'd have to use brackets all over the place. It would not help make things simple.
Yes a nice property of BODMAS is that there is a great deal of symmetry, you can flip around the terms within the additions/subtractions, and inside the multiplications/divisions, and it makes no difference.
Similar threads
