# A=3i + 2j +k and Bz = -1i +2j +Bk. |A + B| =6. Find Bk

1. Feb 3, 2008

### bmchenry

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

2 vectors are given by A=3i + 2j +k and Bz = -1i +2j +Bk. A + B =6

2. Relevant equations

Find the two possible vlaues of Bz

3. The attempt at a solution

6 = sqrt (4 + 20 +Bzsquared)

I get one number to equal -4.87 but when I solve for the other I get it wrong on the online check

2. Feb 3, 2008

### Kurdt

Staff Emeritus
Is that the absolute value of A + B = 6?

3. Feb 3, 2008

### bmchenry

yes it is the absolute value 6, I thought the answer would be to just take the positive 3.87 and subtract 1 from it but that was wrong online

4. Feb 3, 2008

### Kurdt

Staff Emeritus
Ok well in the equation you've come up with you wouldn't just have B2. When you add the vectors you would end up with a term (1+B)2. What values can B take to make everything under square root add up to 36?

5. Feb 3, 2008

### bmchenry

Did that and for one answer I got -4.87 which when you put it in the () and square it you get 36 or close enough that the online problem accepted the answer, I assumed that for the other number I just needed to take 3.87 and subtract the 1 which left me with 2.87, however when I plug that in it says it is incorrect, where is my mistake?

6. Feb 3, 2008

### Kurdt

Staff Emeritus
Can you formulate the problem in a better way? For example you know that:

$$(B+1)^2 = 12$$

How would you solve that for B.

7. Feb 3, 2008

### bmchenry

Never mind but thanks, I figured it out the answer is 3, but I need help bisecting a vector can you help?

8. Feb 3, 2008

### Kurdt

Staff Emeritus
3? I would not have thought it would be, but the online thing might be accepting answers within certain ranges. It would be best to post a new thread to receive the maximum attention.