Babyslapmafro
  • Babyslapmafro
Could someone please point out my error solving the following problem involving a cross and dot product. Find u (dot) (v x w) u=<2,-3,1> v=<4,1,-3> w=<0,1,5> My answer=-40 Answer in book=80
Mathematics
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SOLVED
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jamiebookeater
  • jamiebookeater
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Babyslapmafro
  • Babyslapmafro
v x w = <8,20,4> u (dot) <8,20,4>=<2,-3,1> (dot) <8,20,4>=-40
anonymous
  • anonymous
$$\vec{v}\times\vec{w}=(5+3,0-20,4-0)=(8,-20,4)\\\vec{u}\cdot(\vec{v}\times\vec{w})=(2,-3,1)\cdot(8,-20,4)=16+60+4=80$$
anonymous
  • anonymous
You messed up the sign of your middle component when computing your cross product!

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Babyslapmafro
  • Babyslapmafro
|dw:1371328349077:dw|
Babyslapmafro
  • Babyslapmafro
@oldrin.bataku
jdoe0001
  • jdoe0001
|dw:1371328740926:dw|
Babyslapmafro
  • Babyslapmafro
oh i see, I didn't know the j-component was subtracted in general
jdoe0001
  • jdoe0001
it isn't, from the cofactors "checkerboard signs" it'd be + - + so the middle term would be negative
jdoe0001
  • jdoe0001
well, multiplied by -1 rather
anonymous
  • anonymous
@Babyslapmafro you can't do a cofactor expansion without using the right signs :-p |dw:1371328696455:dw|

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