# Dependence condition

## Homework Statement

>There are three vector $$\vec a ,\vec b, \vec c$$ in three-dimensional real vector space, and the inner product between them $$\vec a . \vec a=\vec b.\vec b=\vec a.\vec c=1, \vec a.\vec b=0, \vec c.\vec c=4$$ When setting $$x = \vec b.\vec c$$ ,
(dot here means dot product)
answer the following question: when $$\vec a ,\vec b, \vec c$$ are linearly dependent, find all possible values of $$x$$

## Homework Equations

3. The Attempt at a Solution

For dependent condition

\begin{align} (a×b)·c &= 0\\ a·(b×c) &= 0\\ a(bc \sin θ)&=0 \end{align}

So $\theta= 0$ and $\pi$

Then
\begin{align} x&=|b||c| \cos \theta \\ x&=2 \cos \theta \\ \implies x &= 2 \cos 0, x = 2 \cos \pi \\ x &= \mp 2 \end{align}

Am I right?

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You haven't said what you mean by $\theta$ and your equation $a(bc (sin(\theta))=0$ looks suspect to me. Where do you take into account the angle between $\vec b \times \vec c$ and $\vec a$?

You haven't said what you mean by $\theta$ and your equation $a(bc (sin(\theta))=0$ looks suspect to me. Where do you take into account the angle between $\vec b \times \vec c$ and $\vec a$?
$$\theta$$ is angle?
\begin{align}
(a×b)·c &= 0\\
a·(b×c) &= 0\\
a(bc \sin θ)&=0
\end{align}

i think this is cross product? i tried to apply scalar triple product like the one when you find area of parallelogram ? but i wasnt sure

Ray Vickson
Homework Helper
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## Homework Statement

>There are three vector $$\vec a ,\vec b, \vec c$$ in three-dimensional real vector space, and the inner product between them $$\vec a . \vec a=\vec b.\vec b=\vec a.\vec c=1, \vec a.\vec b=0, \vec c.\vec c=4$$ When setting $$\vec x = \vec b.\vec c$$ ,
(dot here means dot product)
answer the following question: when $$\vec a ,\vec b, \vec c$$ are linearly dependent, find all possible values of $$\vec x$$

## The Attempt at a Solution

For dependent condition

\begin{align} (a×b)·c &= 0\\ a·(b×c) &= 0\\ a(bc \sin θ)&=0 \end{align}

So $\theta= 0$ and $\pi$

Then
\begin{align} x&=|b||c| \cos \theta \\ x&=2 \cos \theta \\ \implies x &= 2 \cos 0, x = 2 \cos \pi \\ x &= \mp 2 \end{align}

Am I right?[/B]
(1) Remove the bold font; it looks like you are yelling.
(2) Remove the arrow above $x$, because $x$ is not a vector---it is a scalar.

(1) Remove the bold font; it looks like you are yelling.
(2) Remove the arrow above $x$, because $x$ is not a vector---it is a scalar.
ok, done

Dick
Homework Helper
ok, done
There's no cross product in your given relations. You know $|a|=1$ and rotating all of your vectors by the same amount doesn't change anything. So why not set $\vec a=(1,0,0)$ and see what the relations are telling you in terms of coordinates?

There's no cross product in your given relations. You know $|a|=1$ and rotating all of your vectors by the same amount doesn't change anything. So why not set $\vec a=(1,0,0)$ and see what the relations are telling you in terms of coordinates?
thanks! but why set $\vec a=(1,0,0)$ and set to what? i though i can use cross product to test dependency?

Dick
Homework Helper
thanks! but why set $\vec a=(1,0,0)$ and set to what? i though i can use cross product to test dependency?
The question isn't asking you about dependency, it's asking for a range of values. And like I said, the point to setting $\vec a=(1,0,0)$ is just for definiteness. You could set it to any vector with norm 1. Then put $\vec b=(b_x, b_y, b_z)$ and $\vec c=(c_x, c_y, c_z)$ and see what the relations say in terms of those coordinates.

The question isn't asking you about dependency, it's asking for a range of values. And like I said, the point to setting $\vec a=(1,0,0)$ is just for definiteness. You could set it to any vector with norm 1. Then put $\vec b=(b_x, b_y, b_z)$ and $\vec c=(c_x, c_y, c_z)$ and see what the relations say in terms of those coordinates.
do you mean i can pick any vector that match the inner product?
$\vec a=(1, 0, 0)$ , $\vec b=(0, 1, 0)$ (?)
is it possible when $a.c cos \theta= 1$
$1.2 cos \theta=1$ and find the $\theta$?

Dick
Homework Helper
do you mean i can pick any vector that match the inner product?
$\vec a=(1, 0, 0)$ , $\vec b=(0, 1, 0)$ (?)
is it possible when $a.c cos \theta= 1$
$1.2 cos \theta=1$ and find the $\theta$?
The picture you should have in your head is taking the three vectors and rotating them all together. And yes, since $\vec a$ and $\vec b$ are unit vectors and orthogonal you could choose them that way. But now you can't fix $\vec c$ since it can have a variety of positions with respect to the other two. Just see what the dot product relations tell you about the coordinates of $\vec c$.

The picture you should have in your head is taking the three vectors and rotating them all together. And yes, since $\vec a$ and $\vec b$ are unit vectors and orthogonal you could choose them that way. But now you can't fix $\vec c$ since it can have a variety of positions with respect to the other two. Just see what the dot product relations tell you about the coordinates of $\vec c$.
does this mean that a and b and c is in the same plane? and since a and b are independent to each other, it span the plane? and c is inside it?
so i can write c as linear combination of a and b so c= ka+sb?

Dick
Homework Helper
does this mean that a and b and c is in the same plane? and since a and b are independent to each other, it span the plane? and c is inside it?
so i can write c as linear combination of a and b so c= ka+sb?
I don't see anything in the problem that would indicate the vectors are coplanar. Why do you think that?

I don't see anything in the problem that would indicate the vectors are coplanar. Why do you think that?
it is said that a.b=0 ?
if $\vec a=(1, 0, 0)$ and $\vec b=(0, 1, 0)$ ,it is independent to each other? so c is in the span of two of them since three of them are dependent? and they are on the same plane?

Dick
Homework Helper
it is said that a.b=0 ?
if $\vec a=(1, 0, 0)$ and $\vec b=(0, 1, 0)$ ,it is independent to each other? so c is in the span of two of them since three of them are dependent? and they are on the same plane?
You're told they are three dimensional vectors. Two independent vectors don't span the space. Nothing in the problem indicates the three are dependent.

You're told they are three dimensional vectors. Two independent vectors don't span the space. Nothing in the problem indicates the three are dependent.
it is said that when a, b, c are linearly dependent find possible value of x ?
You're told they are three dimensional vectors. Two independent vectors don't span the space. Nothing in the problem indicates the three are dependent.
it is said in the problem a,b,c are linear dependence
$1.2 cos \theta=1$ ,from here the angle between a and c is 60 degree? since a and b is orthogonal to each other b and c will be 30 degree? am i right?

Dick
Homework Helper
it is said that when a, b, c are linearly dependent find possible value of x ?

it is said in the problem a,b,c are linear dependence
$1.2 cos \theta=1$ ,from here the angle between a and c is 60 degree? since a and b is orthogonal to each other b and c will be 30 degree? am i right?
All right. Sorry, now I see that it does say they are linearly dependent. So almost true. There's another choice of angle that works as well.

All right. Sorry, now I see that it does say they are linearly dependent. So almost true. There's another choice of angle that works as well.
can i use the fact that a and b span the plane and c is inside the plane? so i can express c as linear combination of them altough it is in 3 dimentional?
x=b.c cos theta since theta is 30 , $x=b.c cos 30 = \sqrt 3$ ?

Dick
Homework Helper
can i use the fact that a and b span the plane and c is inside the plane? so i can express c as linear combination of them altough it is in 3 dimentional?
x=b.c cos theta since theta is 30 , $x=b.c cos 30 = \sqrt 3$ ?
That's one possibility. There's another. There are two different positions in the plane where there's a 60 degree angle between $\vec a$ and $\vec c$. Draw a picture.

That's one possibility. There's another. There are two different positions in the plane where there's a 60 degree angle between $\vec a$ and $\vec c$. Draw a picture.
$\vec a=(1,0,0)$ $\vec b=(0,1,0)$ $\vec c=(1,1,0)$ , i choose this point . and clearly c is in span of a and b, can you give me more hint?

ehild
Homework Helper
What is the magnitude of $\vec c$? What can be the components then?

What is the magnitude of $\vec c$? What can be the components then?
i was wrong , $\vec c=(1,1,\sqrt2)$

That's one possibility. There's another. There are two different positions in the plane where there's a 60 degree angle between $\vec a$ and $\vec c$. Draw a picture.
sorry i was wrong let $\vec c=(1,1,\sqrt2)$

What is the magnitude of c$? What can be the components then?$\vec can you point how can i see the angle here?

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ehild
Homework Helper
sorry i was wrong let $\vec c=(1,1,\sqrt2)$
Are the vectors $\vec a (1,0,0)$, $\vec b (0,1,0)$, $\vec c (1,1,√2)$ dependent?

Are the vectors $\vec a (1,0,0)$, $\vec b (0,1,0)$, $\vec c (1,1,√2)$ dependent?
a and b independent but c is dependent to a and b ?