# Vector length problem

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• Aug 24th 2011, 08:17 AM
Artifact
Vector length problem
Hello, how do I solve the following problem?

http://img9.imageshack.us/img9/9532/awdn.jpg
• Aug 24th 2011, 08:29 AM
Plato
Re: Vector length problem
Quote:

Originally Posted by Artifact
Hello, how do I solve the following problem?

Do you know how to find $2\vec{a}-3\vec{b}~?$

If you do know how to do that, the just do it then find the length of the result.

If you do not, please explain why not. Because you are being asked to do somethings that you don't understand.
• Aug 24th 2011, 08:36 AM
Artifact
Re: Vector length problem
so I guess 2a is 2* 29 and 3b is 3*58? Or am I off track?

http://img718.imageshack.us/img718/284/asssom.jpg
• Aug 24th 2011, 08:41 AM
Plato
Re: Vector length problem
Quote:

Originally Posted by Artifact
so I guess 2a is 2* 29 and 3b is 3*58? Or am I off track?
http://img718.imageshack.us/img718/284/asssom.jpg

Quote:

Originally Posted by Plato
Do you know how to find $2\vec{a}-3\vec{b}~?$
If you do know how to do that, the just do it then find the length of the result.

If you do not, please explain why not. Because you are being asked to do somethings that you don't understand.

You did not answer any of those questions!

WHAT IS $2\vec{a}-3\vec{b}=~?$
• Aug 24th 2011, 08:48 AM
Artifact
Re: Vector length problem
ok, I guess I don't know how to find 2a-3b, I thought I had to find the length of a first and then find the length of b. After that I would multiply a with 2 and b with 3. So 2a-3b would be = 58 - 174. But I guess that is the wrong way of doing it.
• Aug 24th 2011, 08:52 AM
Plato
Re: Vector length problem
Quote:

Originally Posted by Artifact
ok, I guess I don't know how to find 2a-3b

Why are you asked to do something you clearly are not prepared to attempt?
That makes no sense.
• Aug 24th 2011, 11:26 AM
Professor111
Re: Vector length problem
Quote:

Originally Posted by Artifact
ok, I guess I don't know how to find 2a-3b, I thought I had to find the length of a first and then find the length of b. After that I would multiply a with 2 and b with 3. So 2a-3b would be = 58 - 174. But I guess that is the wrong way of doing it.

Hi there (=. Before we start let us understand how scalar multiplication and subtraction of vectors works.

Scalar multiplication:

http://upload.wikimedia.org/math/e/7...fe80b5bfdd.png

Subtraction of two vectors
can be geometrically defined as follows: to subtract b from a, place the end points of a and b at the same point, and then draw an arrow from the tip of b to the tip of a. That arrow represents the vector a − b, as illustrated below:
http://upload.wikimedia.org/wikipedi...ubtraction.svg

The difference of a and b is
http://upload.wikimedia.org/math/6/2...6b927567c7.png

now find the resulting vector and find it's magnitude which you seem to know how to do.

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