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by WarHorseLeBron on May 10th, 2011

WarHorseLeBron

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Help answer this question below.

For the three vectors A, B, and C, what is A•(B x C)

A=6i+8j+10k
B=i+2j+3k
C=3i+4j+5k

This is question 5/15 from the diagnostic exam from my old FE review manual I found.

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Answers. 3 helpful answers below.

  • by engineer is Terminator on May 11th, 2011

    engineer is Terminator

    Asker's Pick

    Selected by the asker, WarHorseLeBron. (What's this?)

    This can be done through matrix formula for cross product, but let's do it from scratch using basics

    Let's take the cross product of (B x C) first
    (i+2j+3k) x (3i+4j+5k) = 3 i x i + 4 i x j + 5 ixk + 2x3x jxi + 2x 4 j xj + 2x5 x j x k + 3 x 3 x k x i + 3 x 4 x kx j + 3 x 5 x k x k

    ix i = jxj = kxk = 0


    ixj=k jxk=i and kxi=j and if jxi=-k kxj=-i and ixk = -j

    So we have, 4k-5j-6k+10i+9j-12i

    =-2i +4j-2k

    Taking dot product with A=6i+8j+10k


    (6i+8j+10k).(-2i +4j-2k) = -12 + 32-20 = 0

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  • by Spinne on May 10th, 2011

    Spinne

    I did it in my head, but I'm pretty sure it's right. The only thing I'm not sure about is what FE stands for?

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  • by Spinne on May 10th, 2011

    Spinne

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