Maths questions

Nerusy · 6 Sep 2011 at 18:04

Mr Jack said:
You can't slop between units and expect your maths to work.

is that it then, even though £1/4 = 25 pence is correct, sqrt(£1/4) = sqrt(25pence) is not correct?

Camarilla · 6 Sep 2011 at 18:11

Nerusy said:
is that it then, even though £1/4 = 25 pence is correct, sqrt(£1/4) = sqrt(25pence) is not correct?

Yep, pretty much. By changing units, you go from sqrt(0.25) to sqrt(25).

Nerusy · 6 Sep 2011 at 18:16

Camarilla said:
Yep, pretty much. By changing units, you go from sqrt(0.25) to sqrt(25).

I go from sqrt(£0.25) to sqrt(25pence) though £0.25 = 25 pence. It is defined.

You're taking £0.25 = 25pence which correct and square root the whole thing. Why would it sudden't not be equal once you square root the whole thing?

Mr Jack · 6 Sep 2011 at 18:21

Nerusy said:
I go from sqrt(£0.25) to sqrt(25pence) though £0.25 = 25 pence. It is defined.

You're taking £0.25 = 25pence which correct and square root the whole thing. Why would it sudden't not be equal once you square root the whole thing?

Units are altered too

sqrt(£0.25) = sqrt(£)sqrt(0.25)
sqrt(25pence)= 5 sqrt(pence)

No I don't know what sqrt pounds or pence are either...

The same thing will happen if you try and mess around with units in a calculation with lengths or weights or volumes or whatever in it.

Mr Jack · 6 Sep 2011 at 18:24

Actually thinking about it more, sqrt(£) and sqrt(pence) does make sense, to convert back you need to shift your units. £1=100p so you need to apply the alteration in the square root of the unit too

sqrt(£0.25) = sqrt(25p) = 5 sqrt(p) = 5 * sqrt(100) * p = 50p = £0.5

QuagmireUK · 6 Sep 2011 at 18:38

Square root of a decimal is not the same as the square root of an integer. So, by changing the units halfway through you are corrupting your equation.

oh, and OP, ignore all this and GL with your exam

Camarilla · 6 Sep 2011 at 18:47

Nerusy said:
I go from sqrt(£0.25) to sqrt(25pence) though £0.25 = 25 pence. It is defined.

You're taking £0.25 = 25pence which correct and square root the whole thing. Why would it sudden't not be equal once you square root the whole thing?

Because square rooting, like all mathematic functions, doesn't take units into account by itself, you have to do it. As far as the maths is concerned, all you've done is sqrt(1/4) and sqrt(25), which gives 0.5 and 5, respectively. Unless you convert one of them to be the same unit as the other, they'll give different answers.

Shayper · 6 Sep 2011 at 18:53

I wish my uni course was this easy

I have to do things like

r=4
E n(n-1)(n^4)(4n-sqrt(4n))

Just be lucky you are still working in just numbers. (oh and I can't find a sigma on my iPhone, so capital e will have to do

Nerusy · 6 Sep 2011 at 18:55

QuagmireUK said:
Square root of a decimal is not the same as the square root of an integer. So, by changing the units halfway through you are corrupting your equation.

oh, and OP, ignore all this and GL with your exam

That is not a correct statement.

In any case Mr Jack is right. When squre rooting one must square room the units of value too. Good job mate

Camarilla · 6 Sep 2011 at 18:58

Nerusy said:
That is not a correct statement.

It is. For example, let's change the unit in a different place. By your equation, because £1/2 = sqrt(£1/4), then £1/4 squared = 25 pence squared. Square both and see what happens.

EDIT: Why the hell did I write 50? Blargh

Nerusy · 6 Sep 2011 at 19:12

Camarilla said:
It is. For example, let's change the unit in a different place. By your equation, because £1/2 = sqrt(£1/4), then £1/4 squared = 25 pence squared. Square both and see what happens.

EDIT: Why the hell did I write 50? Blargh

"decimal is not the same as the square root of an integer" - that is not a correct statement. In fact it's a wierd statement, but it has no relation to the problem here.
As Mr Jack said the original riddle didn't square the units. ie

£1/2 =/= sqrt(£1/4)
£1/2 = sqrt(£^2)(1/4) [pound sign has to have been squared also]

QuagmireUK · 6 Sep 2011 at 19:45

Squared £'s????

Maybe that's the way out of the financial crisis :rolleyes:

weringo · 6 Sep 2011 at 20:05

Here's one to blow your mind - 1m squared x 1m squared = 1m squared. But 100cm squared x 100cm squared = 10,000cm squared

:eek:

Nerusy · 6 Sep 2011 at 20:06

QuagmireUK said:
Squared £'s????

Maybe that's the way out of the financial crisis

Well rolling eyes sure as hell is not helping

I just gave you the example. The key is to substitute £=100p (p-pence).

weringo said:
Here's one to blow your mind - 1m squared x 1m squared = 1m squared. But 100cm squared x 100cm squared = 10,000cm squared

Well yeah that's correct 1m^2 = 10,000cm^2. think of it this way 1m=100cm, square both sides you get 1x1m^2 = 100x100cm^2

SuperTronNarwhal · 6 Sep 2011 at 20:10

Im using this site to do maths homework

Srengam · 6 Sep 2011 at 20:20

4/5=80%

It's as simple as that.

weringo · 6 Sep 2011 at 20:26

Nerusy said:
Well yeah that's correct 1m^2 = 10,000cm^2. think of it this way 1m=100cm, square both sides you get 1x1m^2 = 100x100cm^2

Thanks for that

ps3ud0 · 6 Sep 2011 at 20:32

Mr Jack said:
There are tricks for working out whether numbers are divisible by 3, 6 or 9 also but sometimes the best thing is just to try some numbers and see what works. Just make sure you always divide both sides by the same number. Don't worry about making it in a single step, just keep breaking it down until you can't see a way to break it down anymore.

If the digits in a number add up to 9 or a multiple of it then it can be divided by 9.

i.e. 81 = 8+1 = 9 or 6778926 = 6+7+7+8+9+2+6 = 45 (a multiple of 9) = 4+5 = 9

Hope that made sense

ps3ud0 :cool:

Destination · 6 Sep 2011 at 20:35

bazaarboy said:
you're 30 years old and struggling to work out 20% of 420?

Least he is struggling and trying.
Good work fella, keep it up, keep working for the exam, the end goal is the priority.

cainer · 6 Sep 2011 at 20:40

nayan89 said:
yeah it's 80%

all you do is 4/5 of 420, which gives you 336.

336/420 = 0.8 (80%)

no.

time = 100%
4/5 of the time = 80%