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

[bmchenry]

Homework Statement

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

Homework Equations

Find the two possible vlaues of Bz

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

 

[Kurdt]

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

 

[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

 

[Kurdt]

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?

 

[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?

 

[Kurdt]

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.

 

[bmchenry]

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

 

[Kurdt]

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.