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

• March 28th 2010, 09:15 AM
Quixomatic
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 (Nod)

-Quixotic

Attachment 16111
• March 28th 2010, 09:48 AM
snaes
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?
• March 28th 2010, 09:54 AM
skeeter
Quote:

Originally Posted by Quixomatic
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 (Nod)

-Quixotic

Attachment 16111

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.
• March 28th 2010, 10:17 AM
Quixomatic
@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!
• March 28th 2010, 11:05 AM
snaes
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 $y(0)=2$
• March 28th 2010, 11:48 AM
Quixomatic
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! (Nod)

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.
• March 28th 2010, 02:13 PM
mr fantastic
Quote:

Originally Posted by Quixomatic
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).
• March 28th 2010, 02:52 PM
Quixomatic
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 (Nod)
• March 28th 2010, 06:45 PM
mr fantastic
Quote:

Originally Posted by Quixomatic
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 (Nod)

In my opinion it's pointless providing 'practice questions' without providing answers.