# Dot and cross product?

## Homework Statement

A=(x,3,1) ,B=(x,-x,2)
Determine the value of x if the vector perpendicular to A and B is given by C=(10,-4,-4)

## The Attempt at a Solution

Find A cross B , let A cross B be D . Then D cross C = zero (since they are perpendicular to both A and B) . This sounds logic but there will not be an answer for x. What is wrong?
Then I tried to use dot product A dot C=0, and B dot C=0. Yet the answer of two x are different.

Related Calculus and Beyond Homework Help News on Phys.org
LCKurtz
Homework Helper
Gold Member
You haven't shown your calculations. But perhaps there is no real value of x that works. Perhaps the problem is mis-printed or copied incorrectly.

You haven't shown your calculations. But perhaps there is no real value of x that works. Perhaps the problem is mis-printed or copied incorrectly.
So the both ways to solve the problem are correct?

LCKurtz
Homework Helper
Gold Member
So the both ways to solve the problem are correct?
I didn't say that. I haven't worked the problem; that's your job. If you want me to comment on what you did you need to show your work.

The first way

#### Attachments

• 130.4 KB Views: 283
Sorry a, my camera not good.

Please look at the picture, I don't know why I can't upload more picture.
The second way, A dot C = 0 , x= 1.6
B dot C = 0 , x= 0.4
Thanks

Last edited:
LCKurtz
Homework Helper
Gold Member
I won't try to read upside down. But if your arithmetic is correct and you get different values for ##x##, that would tell you there is no solution. Like I said earlier, perhaps the problem is mis-printed or copied incorrectly.

Thanks

I would like to remark that your approach with the cross product is over-complicated. Once you found C = A x B, and assume that C is parallel with D, you can say C = k D, where k is some number. You do not need to cross-multiply C and D, you just need to confirm that C = k D is possible.

• 1 person
I would like to remark that your approach with the cross product is over-complicated. Once you found C = A x B, and assume that C is parallel with D, you can say C = k D, where k is some number. You do not need to cross-multiply C and D, you just need to confirm that C = k D is possible.
Really easier. But I think the question have some problems. Thank you.