Did my book do this wrong? (Vector Cross Product)

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Discussion Overview

The discussion revolves around a potential error in a vector cross product calculation presented in a mathematics book. Participants analyze the steps of the calculation and question the correctness of the results shown in the book, focusing on specific rows of the vector cross product and their corresponding values.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant suggests that the first row of the cross product should be calculated as (3)(8) - (4)(-5) instead of (3)(8) - (-4)(-5), proposing that this would yield the same result of 44.
  • Another participant agrees that the book is incorrect, providing their own calculation of the cross product and stating that it results in (44, 0, -11), which they argue reflects poorly on the author and proof-reader.
  • A third participant echoes the initial confusion, reiterating the same points about the first and second rows and questioning the final result, suggesting that the author may have intended different values.
  • There is mention of at least two misprints or errors in the example, with a proposed corrected calculation presented in matrix form.

Areas of Agreement / Disagreement

Participants generally agree that there are errors in the book's presentation of the vector cross product, but they do not reach a consensus on the exact nature of the errors or the correct calculations.

Contextual Notes

Limitations include potential missing assumptions about the notation used in the book and the specific values intended by the author, which are not fully resolved in the discussion.

Pindrought
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Reading a book about 3d math, and I am confused as to what happened on this Vector Cross Product problem. I'm thinking there was just an error that wasn't caught.

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For the first row, instead of (3)(8)-(-4)(-5) shouldn't it have been (3)(8)-(4)(-5) and had the same displayed result of 44?
And for the second row, instead of (-4)(2)-(1)(8) shouldn't it have been (4)(2)-(1)(8) and had the result of 0?
For the last row, shouldn't the final result be -11?

Thanks!
 
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The book is indeed wrong:

$(1,3,4) \times (2,-5,8) = ((3)(8) - (4)(-5), (4)(2) - (1)(8), (1)(-5) - (3)(2))$

$= (44,0,-11)$ which speaks somewhat ill of the original author and proof-reader of your text.
 
Pindrought said:
Reading a book about 3d math, and I am confused as to what happened on this Vector Cross Product problem. I'm thinking there was just an error that wasn't caught.

For the first row, instead of (3)(8)-(-4)(-5) shouldn't it have been (3)(8)-(4)(-5) and had the same displayed result of 4?
And for the second row, instead of (-4)(2)-(1)(8) shouldn't it have been (4)(2)-(1)(8) and had the result of 0?
For the last row, shouldn't the final result be -11?

Thanks!
There are at least two misprints/errors in the example. It looks as though the author intended to write $$\begin{bmatrix}1\\3\\ {\color{red}-}4 \end{bmatrix} \times \begin{bmatrix}2\\-5\\ 8 \end{bmatrix} = \begin{bmatrix}(3)(8) - (-4)(-5)\\(-4)(2) - (1)(8)\\ (1)(-5) - (3)(2) \end{bmatrix} = \begin{bmatrix}4\\-16\\ {\color{red}-11} \end{bmatrix}.$$
 
Thank you very much!
 

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