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Topic: solving X^3+8x^2 (Read 2521 times) |
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lilas224
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solving X^3+8x^2
« on: Oct 12th, 2007, 6:57pm » |
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my question would be how to solve
X^3+8x^2
and my second
The number of bacteria in a refrigerated food product is given by , N(T)=25T^2-120T+57,5<T<35 where is the temperature of the food.
When the food is removed from the refrigerator, the temperature is given by , T(t)=3t+1.7where is the time in hours.
Find the composite function :
\
N(T(t))=?
and
Find the time when the bacteria count reaches 1856
time needed= ?
please help! either one!
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Michael Dagg
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Re: solving X^3+8x^2
« Reply #1 on: Oct 12th, 2007, 7:27pm » |
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We don't know what to do with X^3+8x^2
as you have stated because there is nothing to
solve. If you may mean it is =0 and then that fact
is pretty simple.
I believe you have made some typos in your
subsequent questions.
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| « Last Edit: Oct 12th, 2007, 7:28pm by Michael Dagg » |
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Regards,
Michael Dagg
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towr
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Re: solving X^3+8x^2
« Reply #2 on: Oct 13th, 2007, 2:49am » |
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on Oct 12th, 2007, 6:57pm, lilas224 wrote:my question would be how to solve
X^3+8x^2 |
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If you want to solve for it being 0, it's rather easy, x=0 and x=-8 are solutions. If you want it solved in another way, you'd have to specify.
Quote:The number of bacteria in a refrigerated food product is given by , N(T)=25T^2-120T+57,5<T<35 where is the temperature of the food.
When the food is removed from the refrigerator, the temperature is given by , T(t)=3t+1.7where is the time in hours.
Find the composite function :
N(T(t))=? |
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As best as I can tell, you'd just want to replace each T with (3t+1.7), so you'd get N(T(t)) = 25(3t+1.7)^2 - 120(3t+1.7) + 57, for 5 <(3t+1.7)< 35
You can simplify it further of course, but I don't want to have all the fun.
Quote:Find the time when the bacteria count reaches 1856
time needed= ? |
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Solve the quadratic N(T(t)) = 1856, you'll get two values for t, pick a positive real one (I suspect they'll both be real, and only one will be positive)
So is this homework?
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Wikipedia, Google, Mathworld, Integer sequence DB
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FiBsTeR
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Re: solving X^3+8x^2
« Reply #3 on: Oct 13th, 2007, 8:13am » |
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on Oct 13th, 2007, 2:49am, towr wrote:
Google seems to think so; a user 'lilas224' has posted this question here as well as at Yahoo! Answers.
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temporary
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Re: solving X^3+8x^2
« Reply #4 on: Jan 24th, 2008, 10:49pm » |
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on Oct 13th, 2007, 2:49am, towr wrote:
If you want to solve for it being 0, it's rather easy, x=0 and x=-8 are solutions. If you want it solved in another way, you'd have to specify.
As best as I can tell, you'd just want to replace each T with (3t+1.7), so you'd get N(T(t)) = 25(3t+1.7)^2 - 120(3t+1.7) + 57, for 5 <(3t+1.7)< 35
You can simplify it further of course, but I don't want to have all the fun.
Solve the quadratic N(T(t)) = 1856, you'll get two values for t, pick a positive real one (I suspect they'll both be real, and only one will be positive)
So is this homework? |
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I don't think he means as an equation. Perhaps he means find what it's derivative is or what it is in a single term of x.
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ima1trkpny
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Re: solving X^3+8x^2
« Reply #5 on: Jan 24th, 2008, 11:35pm » |
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on Jan 24th, 2008, 10:49pm, temporary wrote:
I don't think he means as an equation. Perhaps he means find what it's derivative is or what it is in a single term of x. |
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Seriously... just be quiet when you don't know what you are talking about. This is just algebra, no calculus or derivatives required.
But since you apparently need hand-holding to see how it is solved... I'll just finished where Towr left off. (Yes, he was correct.)
First, simplify.
N(T(t))=25(3t+1.7)2-120(3t+1.7)+57
becomes
N(T(t))=225t2-105t-74.75
(assuming you can add, subtract and multiply correctly...)
Which you then set equal to 1856...
1856=225t2-105t-74.75
subtract both sides...
0=225t2-105t-1930.75
using the quadratic formula you get t= (105 +/- 1322.384)/450
which simplifies to t= 3.172, -2.705 (I'm just rounding everything to 3 decimal places because at this math class level that is more than suitable accuracy)
and since time must be positive you can eliminate the second answer.
Therefore, at t=3.172 hours you will have 1856 bacteria.
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temporary
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Re: solving X^3+8x^2
« Reply #6 on: Jan 25th, 2008, 6:56am » |
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on Jan 24th, 2008, 11:35pm, ima1trkpny wrote:
Seriously... just be quiet when you don't know what you are talking about. This is just algebra, no calculus or derivatives required.
But since you apparently need hand-holding to see how it is solved... I'll just finished where Towr left off. (Yes, he was correct.)
First, simplify.
N(T(t))=25(3t+1.7)2-120(3t+1.7)+57
becomes
N(T(t))=225t2-105t-74.75
(assuming you can add, subtract and multiply correctly...)
Which you then set equal to 1856...
1856=225t2-105t-74.75
subtract both sides...
0=225t2-105t-1930.75
using the quadratic formula you get t= (105 +/- 1322.384)/450
which simplifies to t= 3.172, -2.705 (I'm just rounding everything to 3 decimal places because at this math class level that is more than suitable accuracy)
and since time must be positive you can eliminate the second answer.
Therefore, at t=3.172 hours you will have 1856 bacteria. |
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What I need isn't hand-holding, what I need is specification to the x^3+8x^2. What am I supposed to do with it?
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ima1trkpny
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Re: solving X^3+8x^2
« Reply #7 on: Jan 25th, 2008, 7:22am » |
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on Jan 25th, 2008, 6:56am, temporary wrote:
What I need isn't hand-holding, what I need is specification to the x^3+8x^2. What am I supposed to do with it? |
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And that was already explained if you care to read... whoever posted that was not at a point where they understood calculus. The only method to solve the equation would be to set it equal to zero. At which point even you should be able to solve it.
But if you'd like to see derivatives...
f(x)=x3+8x2
f'(x)= 3x2+16x
f"(x)=6x+16
f"'(x)=6
Anything else you'd like to see done with the function?
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| « Last Edit: Jan 26th, 2008, 8:09am by ima1trkpny » |
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ThudnBlunder
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Re: solving X^3+8x^2
« Reply #8 on: Jan 25th, 2008, 7:40am » |
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on Jan 25th, 2008, 7:22am, ima1trkpny wrote:
But if you'd like to see derivatives...
f(x)=x3+8x2
f'(x)= 3x2+8
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Oops, f'(x) = 3x2 + 16x
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ima1trkpny
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Re: solving X^3+8x^2
« Reply #9 on: Jan 25th, 2008, 3:38pm » |
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on Jan 25th, 2008, 7:40am, ThudanBlunder wrote:
Oops  fixed... that's what I get for being in a hurry. Thanks 
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Icarus
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Re: solving X^3+8x^2
« Reply #10 on: Jan 25th, 2008, 6:35pm » |
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on Jan 24th, 2008, 11:35pm, ima1trkpny wrote:| Seriously... just be quiet when you don't know what you are talking about. This is just algebra, no calculus or derivatives required. |
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I'm sorry, but I don't think this was called for. Temporary appears to me to have been doing his best to suggest his ideas on what was meant. He was not impolite or impolitic in his reply. Even if you don't agree that it was wise, rudeness is not necessary.
I agree that the nature of the problems and wording are indicative of pre-calculus mathematics. But temporary may not have the experience of teaching/tutoring mathematics at this level to recognize this fact, and assumed that in this forum the level of the question would be at least a little higher.
It seems to me that a lot of bad feelings raised by an apparently different poster have been transferred to temporary because of his expressed support for said poster. I personally don't care for nonsense-trolls either. Some of the longer members may recall one several years ago who went around posting mathematics-related gibberish on every thread, until I finally got so fed up with it that I started deleting the posts. After a day-long fight, he finally tired of the game and moved on.
But it seems to me that temporary does not fall in this category. If he is mistaken or takes things too lightly, well, I can with very little effort find numerous posts from each of us where the same can be said. If he has been rude, we have been rude to him as well. If we disagree about the worth of his questions, I've seen considerably worse posted that have not received the ire that his have.
You don't have to like him. But outright rudeness is not appropriate.
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ThudnBlunder
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Re: solving X^3+8x^2
« Reply #11 on: Jan 25th, 2008, 7:57pm » |
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on Jan 25th, 2008, 6:35pm, Icarus wrote:
It seems to me that a lot of bad feelings raised by an apparently different poster have been transferred to temporary because of his expressed support for said poster. |
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ima1trkpny obviously believes that Temporary is the net pest srn347, and I tend to agree with her. There seem to be too many similarities, eg. multitudinous posts in one session, baldly stating quirky, half-baked ideas as established facts, obsession with infinity, same log on time, brevity of posts, unrequired, unprovoked defence of srn347, etc. If so, her tone is understandable. Although his attitude does seem to have improved, it is a measure of the similarity that even pavlovian towr has sworn not to react to his posts again. 
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| « Last Edit: Jan 26th, 2008, 5:38am by ThudnBlunder » |
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ima1trkpny
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Re: solving X^3+8x^2
« Reply #12 on: Jan 26th, 2008, 6:42am » |
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on Jan 25th, 2008, 6:35pm, Icarus wrote:
I'm sorry, but I don't think this was called for. |
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Fair enough. I may have been a bit too harsh and for that I apologize.
Quote:| I agree that the nature of the problems and wording are indicative of pre-calculus mathematics. But temporary may not have the experience of teaching/tutoring mathematics at this level to recognize this fact, and assumed that in this forum the level of the question would be at least a little higher. |
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Again... fair enough. But this is the reason I'm quite convinced srn347/Temporary are indeed the same person. Both problems were of the kind taught in Algebra II/Pre-calc. I've never been a math teacher and only did tutoring in high school, but I remember learning this at about 14 years old... and would have recognized the problem as doable with the math I'd learned up to that point. Temporary can't recognize the problem which sounds to me like either he hasn't yet studied them or hasn't done it for long. In which case his age matches up with Srn347's who was 13.
Quote:| You don't have to like him. But outright rudeness is not appropriate. |
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I will endeavor to be more polite but I would like to see a little more thought put into temporary's posts... or better yet, ask questions because most of the people here are willing to explain things and help him learn.
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