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Math Help - Lost on Practice Test, teacher gave us test without answers to probems

  1. #1
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    Lost on Practice Test, teacher gave us test without answers to probems

    My professor gave our class a practice test. However, they not only didn't show how each problem is done, they also didn't even give us what the correct answers were. I'm really lost and have a test tomorrow. If anyone could give step by step solutions to the problems I would really really appreciate it. I've been working on these for the last 3 days and just can't seem to get anywhere.

    Here is an image of all of the problems I have been working on. Please if anyone can post solutions to any of the problems at all, it would be a great deal of help

    -Quixotic

    Lost on Practice Test, teacher gave us test without answers to probems-practice-test.jpg
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  2. #2
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    Here is number one, Ill try and do some more...

    I used a program so I could do this faster for you, basically for number 1, what you need to do is this:

    To see if what they gave you is a solution: Find its derivatives (y , y1, y2 on my picture) and plug those in for y, y', and y''. Then simplify the expression. Upon doing so it should equal zero, which it does for part c.

    EDIT: I should explain my work i think...as you can see I put the value in for y that they gave us...output 99
    Next line, I set y1 equal to the derivative of y (so it is like y')....output 100
    3rd line, I set y2 equal to the derivative of y1 (so its like y'', its just called something different because im using a program)...output 101
    4th, I subbed in all of the values that we just found into the origional differential equation. I had the program simpify the answer.....output 102

    Hopefully this makes sense?
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  3. #3
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    Quote Originally Posted by Quixomatic View Post
    My professor gave our class a practice test. However, they not only didn't show how each problem is done, they also didn't even give us what the correct answers were. I'm really lost and have a test tomorrow. If anyone could give step by step solutions to the problems I would really really appreciate it. I've been working on these for the last 3 days and just can't seem to get anywhere.

    Here is an image of all of the problems I have been working on. Please if anyone can post solutions to any of the problems at all, it would be a great deal of help

    -Quixotic

    Click image for larger version. 

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    why don't you post your work on the ones giving you problems so that someone will be better able to help you?

    Only a very few people knowledgeable enough to complete this will want to do this practice test just to post the answers. Someone may offer hints on a few. like ...

    for #1, find y' and y'' for I, II, and III . sub them into the DE to see if it makes the DE true.

    ... but the best way is to post a few (no more than 3 problems per post) that you attempted (show something) and are unsure of the correctness of your result.
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  4. #4
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    @Snaes,
    Thankyou so so much for your solution, no more explanation is needed. Because of your work I was able to do the problem and I now understand what to do in those types of problems. Once again I greatly appreciate your help and anyone who helps in the future.

    @Skeeter,
    Thankyou for your post I know I shouldn't post so many problems at one time. However I was absent from my class for an entire week due to a scheduled surgery on my ankle. Although my teacher was aware that I would be absent and took preparations such as giving me notes from earlier years, I still lack experience with these problems. I don't necessarily need a solution to every problem, but each problem that I did post I do need help on. So hints or solutions or some sort of morale booster such as a goodluck or what not will still be very much appreciated

    For anyone who is willing to post any help, I've already solved #2 with a java program and no longer require assistance with that one. Luckily no problems like that will be on the real test.

    EDIT: I've also just solved #3 with help from someone on AIM! woohoo!
    Last edited by Quixomatic; March 28th 2010 at 09:20 AM. Reason: solved problem #3
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  5. #5
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    EDIT: NUmber3...Dang i just saw that you got this one....
    ONLY 2 of these satisfy:
    y(0)=2
    This is like: y(x)= "y value"
    Put in 0 for x and y should* be 2 if this satisfies the condition.
    These are C) and E).


    I tried guess and check:
    \dfrac{dy}{dx}=5x^{4}y

    Now find /dfrac{dy}{dx} for the given choices and see which one of them is a solution (the left and right side of the equation should be equal).

    I tried C) first (which didnt work out):
    y=2e^{x^{4}}
    y'=4x^{3} * ex^{4}
    y'=8x^{3} e^{x^{4}}
    Now that I have dy/dx or y', I can see it does NOT equal 5x^{4} y, where y is y=2e^{x^{4}}.
    8x^{3}e^{x^{5}}\not= 5x^{4}*2e^{x^{5}}

    This didnt work, so I looked at how I chose wrong...Next i tried e):
    Same process:
    y=2e^{x^{5}}
    y'=5x^{4} * 2e^{x^{5}}
    y'=10x^{4}e^{x^{5}}
    10x^{4}e^{x^{5}}= 5x^{4}*2e^{x^{5}}
    Yes this works for the equation AND y(0)=2
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  6. #6
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    Wow Snaes thankyou so much for posting that about #3, what I had done is just separate the equation then integrate both sides giving me answer "d" which was y=e^(x^5)+c I should probably learn to read the problems before I just start going at it. Thanks again for solving this problem as well!

    I believe I have just finished problems 4 and 5 but I'm not positive. I got answer A for #4 and answer B for #5.
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  7. #7
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    Quote Originally Posted by Quixomatic View Post
    My professor gave our class a practice test. However, they not only didn't show how each problem is done, they also didn't even give us what the correct answers were. [snip]
    Ask for that material - I would have thought it's part of their job to provide it (or at least provide answers if not solutions).
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  8. #8
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    I've already emailed the teacher and used blackboard in order to communicate with the entire class. The professor never gave out any sort of solutions to the practice test. It was meant to prepare you for the format of the test itself. I personally need to see the solutions or at least the answers so I can be completely ready for the test. Good suggestion though
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  9. #9
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    Quote Originally Posted by Quixomatic View Post
    I've already emailed the teacher and used blackboard in order to communicate with the entire class. The professor never gave out any sort of solutions to the practice test. It was meant to prepare you for the format of the test itself. I personally need to see the solutions or at least the answers so I can be completely ready for the test. Good suggestion though
    In my opinion it's pointless providing 'practice questions' without providing answers.
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