Homework Help: Angle Between Two Vectors

1. Apr 29, 2010

coolamasta

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

http://www.flickr.com/photos/coolamasta/4563408911/ [Broken]

2. Relevant equations

v . w = ||v|| ||w|| cos q

3. The attempt at a solution

i calculate until i got the lambda value is 8/3. then i compare it with 21 and the answer is wrong. i dunno what to do. please help. tq.

Last edited by a moderator: May 4, 2017
2. Apr 29, 2010

D H

Staff Emeritus

Please post the question here, without pictures.

Last edited by a moderator: May 4, 2017
3. Apr 29, 2010

coolamasta

The angle between two vectors v1 = (6,3,-2) and v2 = (-2,lambda,-4) is arccos (4/21). Find the value(s) of lambda. i hope u can understand the questions. thanks in advance.

Last edited by a moderator: Apr 29, 2010
4. Apr 29, 2010

coolamasta

http://1.bp.blogspot.com/_2w6xphCWyPE/S9ntDt8niJI/AAAAAAAAAE8/6KISjbB_YCU/s1600/soalan1.jpg [Broken]

http://4.bp.blogspot.com/_2w6xphCWyPE/S9ntPNm08TI/AAAAAAAAAFE/8w4Fsfu8ISA/s1600/soalan3.jpg [Broken]

my lecturer gave me this assignments 3 days ago. i spent 2 days already tried to solving it but still cant solve it. i manage to solve 3 out of 5 of his questions but the other two i cant. i went to see him this morning n he's not giving me any tips or solution at all.. i tried to find example questions in his tutorial books but did not find anywhere near this questions. please help me. thanks in advance.

Last edited by a moderator: May 4, 2017
5. Apr 29, 2010

D H

Staff Emeritus
FIrst off, please do learn to use proper English. Sentences start with a capital letter. So does "I".'tq' is not a word. Stop with the silly txtspeak.

OK, had to get that off my chest.

Now as far as your calculated lambda value, show some work. It's a bit hard to figure out where you went wrong when you didn't show us what you did.

6. Apr 29, 2010

coolamasta

Sorry for that. This is my work so far.

http://2.bp.blogspot.com/_2w6xphCWyPE/S9n0ILgCiXI/AAAAAAAAAFU/W1NtWNirRBc/s1600/IMG00296-20100430-0459.jpg [Broken]

Last edited by a moderator: May 4, 2017
7. Apr 29, 2010

D H

Staff Emeritus
Correction post first:

The problem statement has v1=(6,3,-2). You stated v1=(6,3,2) in post #3. I edited post #3 to reflect the statement.

8. Apr 29, 2010

D H

Staff Emeritus
If $\theta = \arccos(4/21)$, what is $\cos \theta$?

9. Apr 29, 2010

coolamasta

Is it there where i went wrong?

10. Apr 29, 2010

D H

Staff Emeritus
Never mind, I see that you get that.

What you did wrong was to assume that your work meant that

$$4=-12 + 3\lambda +8$$

and the same for the denominator.

Look at it this way: 3/6=1/2. certainly neither of 3=1 nor 6=2 is true. Now let's put a variable in there: x/6=1/2. What is x? The solution is found by multiplying both sides by 6, yielding x=6/2=3.

In this problem you have a radical, so at some point it might help to square both sides.

A problem can arise when you do this. Just because you have a solution to the squared expression does not mean you have a solution to the original expression. For example, -1=1 is obviously false, but squaring both sides yields 1=1, obviously true. You will need to double-check your solutions.

11. Apr 29, 2010

coolamasta

This is the only solution that i know. I've asked everyone around me and they told me they also couldn't find the answer for it. What should I do now?

12. Apr 29, 2010

D H

Staff Emeritus
What you did was wrong back in elementary school days when you had to learn fractions.

What is the first step often used to solve a problem involving fractions? You want to change the problem to one not involving fractions.

13. Apr 29, 2010

coolamasta

I think i get it now. I'll try it again. Thank you.

14. Apr 29, 2010

D H

Staff Emeritus
Good luck! Let us know if you run into problems.