# Math Help - Vectors (ii)

1. ## Vectors (ii)

Can someone tell me how to arrive to the answer. 1/2(u + 3v)

Thanks

2. Originally Posted by r_maths

Can someone tell me how to arrive to the answer. 1/2(u + 3v)

Thanks
From the diagram it looks to me like BD = v... Something doesn't "add up" (if you don't mind the pun.)

-Dan

3. Ah, sorry.

BC not BD.

(awful pun lol)

4. Originally Posted by topsquark
From the diagram it looks to me like BD = v... Something doesn't "add up" (if you don't mind the pun.)
I love that pun! Much funnier than all puns I make.

5. Am I right to say the arrows play a part into getting the answer?

6. Originally Posted by r_maths
Am I right to say the arrows play a part into getting the answer?
Yes. If you have the arrows backwards then you different answers when you find the vector sum.

7. Originally Posted by r_maths

Can someone tell me how to arrive to the answer. 1/2(u + 3v)

Thanks
Hello,

I've tried to get a solution of this problem and I failed very successfully

Could it be that the proportion reads:

AD : DC = 2 : 1 (?)

If so, the solution is:

1. AD = u + v

3. BC = -u + AD + DC

BC = -u + u+v + (1/2)*(u+v) = v + (1/2)*u + (1/2)*v = (1/2)*(u + 3v)

8. My apologies, it is AD : DC
-
Two mistakes in just copying out one question

Thanks alot by the way.

9. Originally Posted by r_maths
...it is AC : DC
...
Hi,

now it's me who is really confused - because:

AC = AD + DC. Therefore:

AC/DC = 2 ==> AC = 2*DC ==> AD + DC = 2*DC ==> AD = DC

From my previous post you know that

BC = -u + AD + DC

BC = -u + u+v + u+v = u + 2v