# Homework Help: A few vector problems

1. Jan 13, 2005

### formulajoe

ive been struggling with these for an hour and a half.
they arent hard, i just cant seem to make them work.
the first one is:
A = (5,3,2)
B = (-1,4,6)
C = (8,2,0)

im using this format (x,y,z)

aA+bB+ C = a vector parallel to the y-axis.
so, that means i need to find a vector with only z components right?
well, the answers out of the back of the book are a = -12/7 and b = -4/7.
these dont work out.

heres the second one
A = (a,1,4)
B = (3,b,-6)
C = (5,-2,c)

Im supposed to find a,b, and c so that all three vectors are perpendicular to each other.
that means i should use the dot product with A and B, A and C, and B and C and set the dot product equal to zero for all three. So I did this, and i got answers for a, b and c but they dont work out when i check them.

2. Jan 13, 2005

### vincentchan

if a vector is parallel to y axis, that's means the vector is pointing at y direction.... so what will the x and z component of this vector be? after you have the x and z component.. what you ganna to is write down the equation for x&z component... then you will have 2 equation, with 2 unkn..for the 2nd problem, your method is completely fine... check your algebra again

3. Jan 13, 2005

### formulajoe

okay, i tried this
5a -b + 8 = 1
3a + 4b +2 = 0
this leaves a 1 in x, and a 0 in y

i solved it this way, and got close to the right answer. am i on the right track?

4. Jan 13, 2005

### formulajoe

okay, this way should be right
5a - b = -8
2a + 6b = 0
a = -1.5
b = .5
maybe the answers in the back of the book are wrong?

5. Jan 14, 2005

### Leo32

Hi Formulajoe,

Did you consider the questions of Vincenchan ?
If a vector should be parallel to the y axis, what requirements can you put on the x, y, z coördinates. I'm sure you can work out the algebra to calculate the solutions, but I've got the impression you're a bit in the vague as to what you need to calculate...

Greetz,
Leo

6. Jan 14, 2005

### clive

Hi formulajoe,