# Exam prep

1. Apr 16, 2010

### lorik

1. The problem statement, all variables and given/known data

ok im gonna supply some preps I got from exam(all that i remember) and I want you to estimate the degree of roughness

Find the square roots

1) |z|9+27=0

2)find homogenus system

x+y-c+2z=0
-x+2y+c+z=0
x+y+c+z=0
2x+y+c+2z=0 ------->im not sure but its pretty similar I think the concept could be the same

3)vector A(2,3,1) B(3,2,2) C(1,2,1) find vector D

I DONT WANT YOU TO SOLVE MY EQUATION BUT JUST GIVE OUT THOUGHTS of a possible solution BTW I failed this exam, next one I'll be hoping to get in is in june
2. Relevant equations

3. The attempt at a solution

2. Apr 16, 2010

### annoymage

i'm not sure what the question ask, but i'll try to understand.
and sorry if i have a bad english, ngahaha

square root of what?

maybe the question asking the values of the unknown of the homogeneous system,

if yes,

try to convert them to Ax=B

hmm how do i show A, im not really good in latex, and lazy to find one, but it is something like

( 1 1 -1 2 ) x 0
( -1 2 1 1 ) y = 0
( 1 1 1 1 ) c 0
( 2 1 1 2 ) z 0

something like that ahaha.

then they are many ways to solve it.

i think something is missing on this question,

3. Apr 16, 2010

### Staff: Mentor

Do you have any thoughts?

There is not enough information in your third problem. If all you know is three vectors, and no other information, there's no way to find some other vector.

4. Apr 17, 2010

### lorik

1)of z.
2)can i use row echelon form, like gauss elimination ?
3)I think not !

5. Apr 17, 2010

### annoymage

1. of you mean to find $$\sqrt{z}$$ or z1/2

then

lzl9 = -27

and you have to make z in term of a number
or something like this

$$\sqrt{z}$$ = ????

2. of course you can use gauss elimination method, it think you need to understand that method so you can apply it for some other problems

3. i really think to find D is imposible, how can you find one, there dont have any relation from A,B or C

6. Apr 17, 2010

### lorik

1) Should it be z=square root of 1 square + 27 square = 28

arctang =b/a =27/1 = pi/4 ?????
----> 28(cos pi/4 + isin pi/4)
square root (cos Φ+2kpi/n + isin Φ+2kpi/n)

I need a bit of help here !

2)thanks for clearing that up

3)maybe im hypothesizing dont know ,will look it up.

7. Apr 17, 2010

### Staff: Mentor

1)
The question was probably find all of the roots of this equation. That would be find all nine of the ninth roots (not square roots) of |z|9, where |z|9 = -27.

For 2, yes use row reduction to find the solution of the system of equations.

For 3, we're telling you that you haven't provided enough information.
This is just like if I said x = 2, y = 5, z = -3, find w. Unless we know of some relationship between x, y, z, and w, this is a meaningless question. Same thing with the vectors in the question you asked. There has to be some relationship between all the vectors so that it's possible to find the last vector.