# Differentiation of scalar and vector products

#### gusmas

Joined Sep 27, 2008
239
Hey guys

I have a Tutorial that I need to hand in , in 2 weeks but I am stuck with the following problem:

The question Is: "question.jpg"
The answer according to the notes the lecturer gave us is: 2-2u-9u^2.

When I do the math(see atttached document: "workings.jpg" My answer differ slightly and I can not see the problem in my workings, so hoping you guys can maybe spot a problem or if not maybe the answers he gave us is not correct.(even though I asked him and he could not give me a 100% on a yes or no).

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#### Georacer

Joined Nov 25, 2009
5,182
Is any one of i or j the imaginary unit?

#### Papabravo

Joined Feb 24, 2006
20,399
I don't think so. ijk are commonly used basis vectors for the Cartesian coordinate system.

I like your result. The sign of the $$u^2$$ term in the dot product is positive. I don't see how he got the negative sign.

Last edited:

Joined Jul 7, 2009
1,583
If i, j, and k are the usual Cartesian unit vectors and your $$F \cdot G$$ is the usual dot product, then I get the same answer you do.

#### Georacer

Joined Nov 25, 2009
5,182
Oh, the Cartesian basis vectors, I see. But it seems I 'm a bit too late, as this is already answered.

#### Papabravo

Joined Feb 24, 2006
20,399
Oh, the Cartesian basis vectors, I see. But it seems I 'm a bit too late, as this is already answered.
Well clearly eight eyeballs are better than two

#### gusmas

Joined Sep 27, 2008
239
Wow guys/girls thanks for all the response to this thread, really appreciate it. I don't know how this forum work, but there were more questions to that tutorial , which the answers that were given with it does not agree with my answers, is it ok if I scan in my work and post it in this thread, so you guys can take a look and see if I made a mistake somewhere? I wont ask anyone to work out the problem for me ,I will do all the work. Just want to know if I am on the right path with this work, since a test is coming up aswell and I want to score well in it.

Thanks

#### Georacer

Joined Nov 25, 2009
5,182
Of course you can post your work for verification.

If it is purely math, leave it in the Math forum. Otherwise it might be better suited in the Homework Help section.