
help with math 151
I know i always come here with my math problems, but I really think i understand it better after some of you guys explain it than the teachers. I have two practice exams and i'm stuck with the same type of problem for both of them. It says to use the definition of the derivative as lim h>0. The functions of the two separate problems are
1. f(x)=x^23x
2. f(x)=1/x2
Thank you to anyone who can help me understand this!
the def. of derivative is f(x+h)f(x)/h

Re: help with math 151
1. f'(x) = 2x3
is 2 a)1/(x2) or b)(1/x)2?
a) f'(x) = 1/((x2)^2)
b)f'(x) = 1/(x^2)
If you have to take 165 you're gonna be ****** you had to learn the limit rule.
Last edited by TitanClone; 02172011 at 06:08 PM.

Re: help with math 151
To expand using the definition of derivative, I'll do the first as an example.
lim_(h>0)[f(x+h)  f(x) /h]. Let's drop the limit for now and sub in the function.
[f(x+h)f(x)]/h = [(x+h)^23(x+h)  (x^2  3x)]/h
Expanding the (x+h) terms:
... = [x^2 + 2xh + h^2  3x  3h  x^2 + 3x]/h
Eliminating common factors:
... = [h^2 + 2xh  3h]/h
Factoring out an "h" from the numerator:
... (h)(h+2x3)/h
Canceling the "h" gives h + 2x  3. Taking lim_(h>0) gives 2x3.
The method is always the same. Substitute f(x+h) into the function, subtract f(x), divide by h and take the limit. If it doesn't come out clean, you may have to use L'Hopital's rule.
Last edited by superfan; 02172011 at 06:18 PM.

Re: help with math 151
Originally Posted by superfan
To expand using the definition of derivative, I'll do the first as an example.
lim_(h>0)[f(x+h)  f(x) /h]. Let's drop the limit for now and sub in the function.
[f(x+h)f(x)]/h = [(x+h)^23(x+h)  (x^2  3x)]/h
Expanding the (x+h) terms:
... = [x^2 + 2xh + h^2  3x  3h  x^2 + 3x]/h
Eliminating common factors:
... = [h^2 + 2xh  3h]/h
Factoring out an "h" from the numerator:
... (h)(h+2x3)/h
Canceling the "h" gives h + 2x  3. Taking lim_(h>0) gives 2x3.
The method is always the same. Substitute f(x+h) into the function, subtract f(x), divide by h and take the limit. If it doesn't come out clean, you may have to use L'Hopital's rule.
In high school, my calc teacher mistakenly wrote down "L'Hospital's Rule". Ahh, fun times with math.

Re: help with math 151
Sorry, no help here, but you made me break out into a cold sweat just reading this thread. I have long since forgotten everything I learned in Math 151...
I originally had an Egyptian instructor...the only thing I could understand him say was "quiz every Friday". I got into a different section with an American instructor.
Had to share my war story...

Re: help with math 151
isn't this thing online? Can't you just pay somebody to do it for you?
The one constant through all the years, Ray, has been baseball. America has rolled by like an army of steamrollers. It has been erased like a blackboard, rebuilt and erased again. But baseball has marked the time. This field, this game: it's a part of our past, Ray. It reminds of us of all that once was good and it could be again. Oh... people will come Ray. People will most definitely come.

Re: help with math 151
Originally Posted by thatguy
isn't this thing online? Can't you just pay somebody to do it for you?
lol, no 150 is online and that was a breeze compared to this stuff. And thankfully i don't have to take 165, 151 is the highest i have to go.

Re: help with math 151
Originally Posted by lennon3
lol, no 150 is online and that was a breeze compared to this stuff. And thankfully i don't have to take 165, 151 is the highest i have to go.
Strange. I struggled with Math 150 (twice). I couldn't grasp matricies. I cruised through 151, although I don't remember most of it.

Re: help with math 151
Originally Posted by lennon3
lol, no 150 is online and that was a breeze compared to this stuff. And thankfully i don't have to take 165, 151 is the highest i have to go.
that is what i'm saying. work at a restaurant and bar and use the barter system.
The one constant through all the years, Ray, has been baseball. America has rolled by like an army of steamrollers. It has been erased like a blackboard, rebuilt and erased again. But baseball has marked the time. This field, this game: it's a part of our past, Ray. It reminds of us of all that once was good and it could be again. Oh... people will come Ray. People will most definitely come.

Re: help with math 151
I'm glad I don't have to use anything I learned in Math 151 in my job else I'd be screwed.
How sad is it that I had to drop this class at ISU because the professor teaching it had me totally lost but I took it over the summer at DMACC and some HS Math teacher at Waukee was able to teach it in a manner that it made complete sense? Just goes to show you some people can have all the education and credentials you can possibly get and still not be able to teach the subject worth a crap...

Re: help with math 151
Originally Posted by CYdTracked
I'm glad I don't have to use anything I learned in Math 151 in my job else I'd be screwed.
How sad is it that I had to drop this class at ISU because the professor teaching it had me totally lost but I took it over the summer at DMACC and some HS Math teacher at Waukee was able to teach it in a manner that it made complete sense? Just goes to show you some people can have all the education and credentials you can possibly get and still not be able to teach the subject worth a crap...
also has allot to do with the fact that ISU is a research facility and most of the TA's aren't here to actually teach but have to as part of their scholarship
The one constant through all the years, Ray, has been baseball. America has rolled by like an army of steamrollers. It has been erased like a blackboard, rebuilt and erased again. But baseball has marked the time. This field, this game: it's a part of our past, Ray. It reminds of us of all that once was good and it could be again. Oh... people will come Ray. People will most definitely come.

Re: help with math 151
I'm in 151 right now too. And surprisingly, I'm actually feeling pretty confident about the test tomorrow. My other two math classes at ISU were both online, so it's nice to actually have a lecture to go to (only time that phrase has ever been said?). I only got through them as well as I did because of the three attempts at the test they give us. Didn't feel like I learned a thing in either though. This semester it feels like I am, and I'm actually enjoying calc

Re: help with math 151
pay an engineer 50 bucks to take the test for you.

Re: help with math 151
blah blah blah nerd nerd nerd. You will never need this

Re: help with math 151
Originally Posted by VTXCyRyD
blah blah blah nerd nerd nerd. You will never need this...
...excerpt for the test and this semester.
Let my Fred's Posse Ride: Georges, 3sus Naz, Dustin, Bryce, Abdel, Monte, Matt, and Jameel.
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