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#1 2013-01-23 08:37:04

skylor b
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volume and SA of a cube

express the edge length of a cube as a function of the cubes diagonal length d. then express the surface area and volume of the cube as a function of the diagonal length.

1. a function for the edge length of cube is l(d)=?

2. a function for the surface area of a cube is a(d)=?

3.a function for the volume of a cube is v(d)=?

#2 2013-01-23 08:42:31

bob bundy
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Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

skylor:  You keep posting questions and then give no response to what I'm offering.

Are my posts helpful?

Can you do the problems you have previously posted ?

Bob

View Image: skylor.gif

You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#3 2013-01-23 08:54:38

skylor
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Re: volume and SA of a cube

yes they are thank you

#4 2013-01-23 08:58:23

bob bundy
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Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

OK.  So let's do one at a time, shall we.

How would you work out the length of the diagonal of a cube if the side has length x  ?

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#5 2013-01-23 09:00:52

skylor
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Re: volume and SA of a cube

x^3?

#6 2013-01-23 09:06:38

bob bundy
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Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

That cannot be right.  You are calculating a distance so it should not have a cube term in it.

Take a look at my cube picture below.  (I did this for someone else who posted a problem earlier)

You want the length of AG in terms of AB = x

Use Pythagoras to work out EG in triangle EGH.

Then AG in triangle AEG.

Bob

View Image: cube.gif

You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#7 2013-01-23 09:14:03

skylor
Guest

Re: volume and SA of a cube

ok but im confused because there are no numbers. am I just supposed to give the forumula

#8 2013-01-23 09:17:01

skylor
Guest

Re: volume and SA of a cube

so itd be

l(d) =sqrt(3d)

a(d) = 6s^2

v(d)= d^3

#9 2013-01-23 09:28:19

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

hi skylor

am I just supposed to give the forumula

Yes.

l(d) =sqrt(3d)

a(d) = 6s^2

v(d)= d^3

Now you are getting closer.

d is for the diagonal.  I'm using x for the side.

So the first should be

So now re-arrange this to get x in terms of d.  That will be the formula l(d) for Q1.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#10 2013-01-23 09:34:15

skylor
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Re: volume and SA of a cube

I think it should be sqrt (3x)

#11 2013-01-23 09:34:53

skylor
Guest

Re: volume and SA of a cube

I think I have this one down thank you

#12 2013-01-23 09:43:43

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

I think it should be sqrt (3x)

I disagree.

You calculated like this:

So

There is a 'theory' in physics that covers the 'dimensions' in formulas.

In this case it says

You cannot have

I suggest you post your other answers for these questions so I can check them.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#13 2013-01-23 09:47:16

skylor
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Re: volume and SA of a cube

these are the formulas I am using

l(d) =sqrt(3d)

a(d) = 6s^2

v(d)= d^3

#14 2013-01-23 10:02:40

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

I have seen these already.

I don't think any are correct.

I am trying to lead you through like this.

(i) get the correct formula for d in terms of x.

(ii) re-arrange it to get x in terms of d

(iii) get the correct formula for the surface area in terms of x

(iv) use (ii) to write the surface area in terms of d

(v) get the correct formula for the volume in terms of x

(vi) use (ii) to write the volume in terms of d.

So far I have told you (i) and you haven't got (ii) right.

And you have posted yet another question.

What is the point of this when this question is not yet done?

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#15 2013-01-23 10:11:56

skylor
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Re: volume and SA of a cube

I replaced x with d in those problems. Is that what was wrong?

I posted another question because it is confusing me and I feel like I understand this one enough for class now.

#16 2013-01-23 10:17:11

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

I replaced x with d in those problems. Is that what was wrong?

Yes.  Let's say that x = 5.  Then d = √3 times 5 = 8.66,,,,

So given that d = 8.66,,,, how do you calculate x ?

So what is the formula for x in terms of d ?

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#17 2013-01-23 10:21:27

skylor
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Re: volume and SA of a cube

take the sqrt of that?

#18 2013-01-23 10:23:44

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

√8.66 = 2.94....

You should be saying how to get the answer 5.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#19 2013-01-23 10:25:24

skylor
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Re: volume and SA of a cube

hm ok thanks. I think if I know the formula I should be ok.

#20 2013-01-23 10:29:05

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

So what do you think it is?

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#21 2013-01-23 10:32:12

skylor
Guest

Re: volume and SA of a cube

3.87

#22 2013-01-23 10:44:39

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,312

Re: volume and SA of a cube

That isn't a formula.

x = something with a d in it.

B


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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