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[Christoff Yurievich]

I'm having a problem with a..er..problem in my homework.

C(x) = Sq.rt of (5x²+60)
x(t) = 20t+40

a) find the marginal cost, dC/dx.
b) find dC/dt.
c) inwords, interpret dC/dt.
d) how far will costs be rising when t=4?

I have (a) (I think) it's (b) (and hence c) that are getting me. at least tell me if I have (a) correct - 1 divided by 2(sq.rt.(5x²+60)² times 10x. I hope that came through correctly.

it wouldn't be a problem if there were an example similar to this but the questions before and after this one are not similar at all.

 

[btrboyev]

I can't help you its been way too long..but damn is the puppy cute.


Sorry!

 

[LordPhlegm]

Do you know the chain rule? That's the basic idea behind the first part.

The second, you plug x(t) into x in C(x), and take the derivative in terms of t instead of x.

dC/dt = whatever bs you want, so long as you say it's the rate of change of C (cost?) with regards to t (time?).

Plug 4 into dC/dt, and if it's positive, the change would be positive, so it'd be rising.

I'll look over things, and do it. I'll add an edit or post later.

 

[Christoff Yurievich]

it would look better if the html was working on the board.

and yeah, he's cute.

 

[Christoff Yurievich]

we're studying the chain rule. I suppose if I understood it better it would be easier on me.

thanks for the help.

so it would look like this?

5(20t + 40)suqared + 60 under the sq.rt sign? and then take the derivative of that?

 

[LordPhlegm]

Yes.

I plugged x(t) into C(x), getting:
C(t) = (2000* t^2 + 800 * t + 8060)^(0.5)

Using the chain rule,
dC/dt = (1/2) * (2000* t^2 + 800 * t + 8060)^(-0.5) * (4000 * t + 800)

Plugging t = 4 into the above equation, I got:
+40.386, therefore, C is increasing because it's positive.

My hand math might have been wrong; I'm not used to doing problems by hand anymore, so don't take my work as perfect. The concepts should be good though.

G'luck with the rest of your 'homework' problems. =)

 

[Christoff Yurievich]

you have something that does this automatically? :drool

My test is monday, hopefully I'll get it by then. thanks for the help.

 

[tetsuoxb]

you have something that does this automatically? :drool

My test is monday, hopefully I'll get it by then. thanks for the help.

 

[Ecrofirt]

what the motherfuck is this Titanium shit?

I've got a TI89, but I've never seen one of those things.

 

[tetsuoxb]

The model that replaced the 89.

 

[LordPhlegm]

Yeah, Bune Duggy: purchase a TI-89, and learn how to use it properly. It'll help for years.

I've been using a TI-83 for years, and an 89 would have made so many of my exams a joke.

 

[tetsuoxb]

Yeah, Bune Duggy: purchase a TI-89, and learn how to use it properly. It'll help for years.

I've been using a TI-83 for years, and an 89 would have made so many of my exams a joke.

I second this, especially if you are in high school. It singlehandedly gave me 50 more points on the math section of the sat (630->680). Check to make sure your teacher allows it, but if they arent familiar, dont show your hand that it does symbolic manipulation.

Check before you buy with college professors if you are in college. The 89 was explicitly banned in a few of the classes I was in. It willl help you on homework, but if you cant use it all the time, then you might use it too much as a crutch and screw yourself on tests. It is an answer-checker not an answer-finder.

 

[Ecrofirt]

I'll tell you now that it's responsible for my B in calculus.

I'll pop in a derivative, integral, or limit on that thing, and be able to check my answer in two seconds. It's bliss.

It's funny, because when I took calc in high school, I wasn't supposed to use it, but since I wasn't taking the AP test the teacher didn't care.

I'm in Calc 2 in college now, which is all material I did in high school, and I've learned nothing at all yet. The professor has told us both semesters that we aren't allowed to use graphing calculators on tests, but when he walks around while we're testing kids will have them out and he doesn't say anything. So, I just use the old 89 to check my answers and read the definitions and jazz that I write down in it.

 

[fallout]

Heh, we're not allowed any calculators of any kind in any of my university courses. In 2nd year E&M, we just did orders of magnitude calculations and in calc, the numbers were always fairly easy to work with (which could be a benefit, because if you were getting something that couldn't be done in your head in a few seconds, you probably fucked up).

 

[Ecrofirt]

I don't see the point in not being allowed to use calculators, but that's just me.

I do see the point in not being allowed to use the 89, though. I've used it forever, and it's definitely been a crutch on several occasions.

 

[tetsuoxb]

Heh, we're not allowed any calculators of any kind in any of my university courses. In 2nd year E&M, we just did orders of magnitude calculations and in calc, the numbers were always fairly easy to work with (which could be a benefit, because if you were getting something that couldn't be done in your head in a few seconds, you probably fucked up).

Same for my Multivariate Calculus and Computational Linear Algebra classes.... and I was a Japanese major.

 

[fallout]

I don't see the point in not being allowed to use calculators, but that's just me.

I do see the point in not being allowed to use the 89, though. I've used it forever, and it's definitely been a crutch on several occasions.

Well, on a physics test, it's kind of a waste of time to punch numbers into a calculator. On something like a calc/lin. alg., you shouldn't even need it. So the question arises from the prof, "Why should you need a calculator?"

Same for my Multivariant Calculus and Computational Linear Algebra classes.... and I was a Japanese major.

Not to be an ass (seriously), but it's multivariate.

 

[LordPhlegm]

The TI-89 can handle Root Loci Plots and Bode Plots of Transfer Functions, which would have been very helpful for when I took my signals, systems, and control class. If I had an 89, I would of saved so much time and figured out what was done wrong in advance instead of dwelling on impossible and wrong solutions.

I second this, especially if you are in high school. It singlehandedly gave me 50 more points on the math section of the sat (630->680). Check to make sure your teacher allows it, but if they arent familiar, dont show your hand that it does symbolic manipulation.

Check before you buy with college professors if you are in college. The 89 was explicitly banned in a few of the classes I was in. It willl help you on homework, but if you cant use it all the time, then you might use it too much as a crutch and screw yourself on tests. It is an answer-checker not an answer-finder.

I don't see the point in not being allowed to use calculators, but that's just me.

I do see the point in not being allowed to use the 89, though. I've used it forever, and it's definitely been a crutch on several occasions.

Agreed. Calculators are helpful for getting past tedious work we already know how to do. As Ecro and Tetsuo said, using the 89 as a crutch might not be the best of ideas depending on the classes, but calculators are definitely helpful.

 

[tetsuoxb]

Not to be an ass (seriously), but it's multivariate.

fixed it.... asshole

 

[miyuru]

My friends are always incredulous when they notice I've never had a graphing calculator in my life. I got through high school with my trusty Casio that I found on the floor one day :lol Punching in scientific notation is 100x faster than with TI-83s, as far as I know.

 

[Christoff Yurievich]

Well, on a physics test, it's kind of a waste of time to punch numbers into a calculator. On something like a calc/lin. alg., you shouldn't even need it. So the question arises from the prof, "Why should you need a calculator?"

well, you're a smarter guy than me. I'd die sans calc.

thanks for the recommendations, I'll be looking at the 89 at the bookstore tomorrow. (I'm in college, btw. oh yeah, and it's my first Calc class ever.)

 

[Escape Goat]

All my answers in Calculus were either "true" or "no solution"

 

[pxleyes]

icon menu on the TI89. Lucky bastard.

I love my TI89, and Calc was fun thanks to the games.


I'm sorry I cant help, but I haven't done sq. rt. derivatives in over 2 years. Back then I would have busted this problem up though. Sorry.