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Old 04-22-2012, 04:23 PM   #8381
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Quote:
Originally Posted by funkbass369
can someone show me how to simplify sinx(-cosx/sinx)? I know the answer is -sinx but that doesn't make any sense to me. I would think it would be -cosx since im pretty sure the sinx's cancel.
Umm...it should be -cosx ... why would it be -sinx?
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Old 04-22-2012, 04:32 PM   #8382
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Originally Posted by FishStik
Umm...it should be -cosx ... why would it be -sinx?


that's what the answer says on my homework.
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Old 04-22-2012, 04:38 PM   #8383
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TEACHER ERROR! Never let him hear the end of it.
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Old 04-22-2012, 04:41 PM   #8384
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Originally Posted by FishStik
TEACHER ERROR! Never let him hear the end of it.


He doesn't make the homework. It's done by some textbook company. There seems to be a lot of errors in the answers on our homework though. It's confusing because I never know if I'm doing something wrong or if there is just an error. And then I lose points because the answer key is wrong.

anyways, that's how that problem is done right? just cancel out the sinx's?
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Old 04-22-2012, 04:42 PM   #8385
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Only if x=Pi/4....
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Old 04-22-2012, 04:45 PM   #8386
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Only if x=Pi/4....


the x's are irrelevant in the problem. I just used x's because I didn't feel like figuring out how to type a theta symbol on my keyboard.
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Old 04-22-2012, 04:46 PM   #8387
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Originally Posted by funkbass369
anyways, that's how that problem is done right? just cancel out the sinx's?
Yep. If you don't trust the answer key, Wolfram is your friend. Just don't abuse it.
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Old 04-22-2012, 04:48 PM   #8388
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Originally Posted by funkbass369
the x's are irrelevant in the problem. I just used x's because I didn't feel like figuring out how to type a theta symbol on my keyboard.


Sure. I was half-joking, because sin(Pi/4)=cos(Pi/4). No worries, your answer is the correct one. The sin's cancel. The only place where you have to watch out with canceling functions like that is when the denominator becomes zero. It is often advisable to treat those cases separately. However, in this case I think it's fairly obvious that the limit as x->0 sin(x)/sin(x)=1.
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Old 04-22-2012, 05:36 PM   #8389
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Originally Posted by FishStik
Yep. If you don't trust the answer key, Wolfram is your friend. Just don't abuse it.


thanks a lot for the site. it sucks when you get a C on a test just because the answer key to the homeworks and test reviews are usually wrong.
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Old 04-22-2012, 05:51 PM   #8390
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No probs. You'll find yourself using it more frequently as you get to higher math classes. That 'show steps' button is nothing short of magical.
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Old 04-25-2012, 03:50 PM   #8391
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Doing some science project on concrete and I need help.

Before you ask; no, I don't want you guys to do my homework for me, I'd just like to understand what I'm doing Anyways, here are some questions I need help with...

1. What type of treatment is used in industry while preparing alloys like steel? (Surprsingly had a hard time finding this on google...)

2. How can pH influence the speed of corrosion?

3. (There's a diagram of oxidation reactions)
b) What are the products formed called?
c) Do them etals gain or lose electrons?
d)What atmospheric substance may promote the mobility of electrons?


Again, thanks if any of you guys can help me out with these and hopefully help me actually understand this (I've researched but had a hard time understanding some things ahaha...)
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Old 04-25-2012, 04:18 PM   #8392
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2. Rusting is a redox reaction. Most Redox reactions require H+ or OH-. By Le Chatelier's principle, increasing the reactants will drive the reaction forward. Hope that points you in the right direction.

3. I would prefer to see the diagram before answering this. Are you sure that it is an oxidation process? If it is indeed an oxidation process, the metals will lose electrons. I'm not sure what the products will be called because I'd have to see the reaction in order to predict the products; unless you teacher is looking for a generic term such as Reducing Agent or an Oxidizing Agent.
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Old 04-25-2012, 04:25 PM   #8393
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Originally Posted by metal4eva_22
2. Rusting is a redox reaction. Most Redox reactions require H+ or OH-. By Le Chatelier's principle, increasing the reactants will drive the reaction forward. Hope that points you in the right direction.

3. I would prefer to see the diagram before answering this. Are you sure that it is an oxidation process? If it is indeed an oxidation process, the metals will lose electrons. I'm not sure what the products will be called because I'd have to see the reaction in order to predict the products; unless you teacher is looking for a generic term such as Reducing Agent or an Oxidizing Agent.


The diagrams for 3 are:

2Cu + (oxygen) = 2CuO
4Al + 3(oxygen) = 2Al(2)O(3)
4Fe + 3(oxygen) = 2Fe(2)O(3)



EDIT: Yeah, I'm pretty sure he's looking for a generic term...I mean, it's an easy question, I just probably don't know the term/he expects us to know this/We never learned it/I'm just stupid.

Yeah well, for number 2...I'd think that more oxygen = faster rusting? and since OH- (what makes something basic) contains oxygen...a higher pH makes the rusting process faster? But then if that's true, why would they put steel slabs in concrete...(concrete has a pH of 11)...woiuldn't that mean the steel would rust faster?
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Old 04-25-2012, 04:44 PM   #8394
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Does anyone have any tips for solving trigonometric identity problems?

I can't click with them at all and although they don't make up a large part of my exam I don't want to lose marks unnecessarily and have weak points.
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Old 04-25-2012, 04:54 PM   #8395
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Originally Posted by Jakeylee111
Does anyone have any tips for solving trigonometric identity problems?

I can't click with them at all and although they don't make up a large part of my exam I don't want to lose marks unnecessarily and have weak points.

What sort of problems?

There are (probably) only a handful of identities you are expected to memorise. Try one of them, then go back and try another if it doesn't seem to have helped.
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Old 04-25-2012, 06:23 PM   #8396
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Quote:
Originally Posted by Rancid Ivy
Doing some science project on concrete and I need help.

Before you ask; no, I don't want you guys to do my homework for me, I'd just like to understand what I'm doing Anyways, here are some questions I need help with...

1. What type of treatment is used in industry while preparing alloys like steel? (Surprsingly had a hard time finding this on google...)

2. How can pH influence the speed of corrosion?

3. (There's a diagram of oxidation reactions)
b) What are the products formed called?
c) Do them etals gain or lose electrons?
d)What atmospheric substance may promote the mobility of electrons?


Again, thanks if any of you guys can help me out with these and hopefully help me actually understand this (I've researched but had a hard time understanding some things ahaha...)

I'll try and answer a little more thorough; my last answer was done pretty quickly because I had stuff to do.

1. An alloy is a mixture of elements; steel is a mixture of Iron with another element (usually Carbon). It can thus be reasoned that making an alloy involves mixing one element with another alloying element. In the case of steel, you would mix iron with an alloying element such as Carbon. For a more thorough explanation look at Steel or Steel Making.

2. Corrosion is a pretty general term. The definition of corrosion is Corrosion is the gradual destruction of material, usually metal, by chemical reaction with its environment. Rusting is the most common type of corrosion. A low pH or the presence of salts will speed up the rusting process. There was a really good explanation of the rusting process in my general chemistry text book, however Wikipedia has a good explanation as well.

3. b) I would call those Metal Oxides
c) The metals will lose electrons as oxygen is very electronegative (Steals Electrons).
d) I'm not too sure about what this question is asking. Electron mobility is the electrons moving through a material. Perhaps someone who knows a little more about electricity can answer this question better. If I had to venture a guess, I would say that Oxygen in the air will speed up the redox process, therefore liberating more electrons.

Wikipedia is a great starting place to gather information. Just make sure that what wikipedia says coincides with what your text book says. And never cite wikipedia as a source.
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Old 04-25-2012, 06:31 PM   #8397
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Quote:
Originally Posted by Jakeylee111
Does anyone have any tips for solving trigonometric identity problems?

I can't click with them at all and although they don't make up a large part of my exam I don't want to lose marks unnecessarily and have weak points.


You usually can get them all by remembering:

cos^2+sin^2=1 <=> 1+tan^2=1/cos^2 <=> cot^2+1=1/sin^2
Addition theorems:
cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
sin(a+b)=sin(a)cos(b)+sin(b)cos(a)

A strategy that I like is taking both sides of an identity and changing them until you get a tautology, i.e. 1=1. Although my math teacher didn't accept that as an answer, from there it is easier to work your way back.

Post some sample questions maybe.
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Old 04-25-2012, 07:00 PM   #8398
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Quote:
Originally Posted by keinerniemand
You usually can get them all by remembering:

cos^2+sin^2=1 <=> 1+tan^2=1/cos^2 <=> cot^2+1=1/sin^2
Addition theorems:
cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
sin(a+b)=sin(a)cos(b)+sin(b)cos(a)

A strategy that I like is taking both sides of an identity and changing them until you get a tautology, i.e. 1=1. Although my math teacher didn't accept that as an answer, from there it is easier to work your way back.

Post some sample questions maybe.

Thanks for the help mate, i'll note them down and post some questions I struggle with over the weekend. (Studying for another test at the mo).
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Old 04-25-2012, 07:12 PM   #8399
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EDIT:I have a test on this material tomorrow and I need an A on this test to get an A in the class so I would really, really appreciate it if someone could help me figure this out.

problem is in bold:
Calculate the energies of the following waves (in kilojoules per mole), and tell which member of each pair has the higher value.

An FM radio wave at 99.5 MHz and an AM radio wave at 1250 kHz.


I know the answer to the FM wave is 3.97×10^-5 KJ/mol and the answer to the AM wave is 4.99×10^−7 kJ/mol .

I did this problem before on a homework assignment and got the correct answers but I can't seem to get it right now.

The first formula I have to work with is c=λv where c is the speed of light(3.00*10^8), λ is the wavelength and v is the frequency. The second formula is E=hv where E is energy (in joules), h= Plank's constant(66.626*10^-34) and v is again, the frequency.

Now am I pretty sure I am supposed to be using only the second formula here, but i keep getting the wrong answers. And I'm pretty sure I'm converting everything to the correct unit. (v has to be in hertz, λ has to be in meters).

so can anyone help me out with this?
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Old 04-25-2012, 08:14 PM   #8400
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E = hv

h = 6.626 * 10^-34

That's energy per...electron I'm guessing? (Otherwise how do you have a mole of waves? wtf? lol)

Take that number and multiply by Avogadro's number. Voila, kJ/ mol .
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