# Don't understand what the book means, a tensor thing...

1. Jul 12, 2016

### Oz123

1. The problem statement, all variables and given/known data
Right, so it's not really an assignment or anything, just confused of what a book says. the book is "mathematical methods for physicists." The screenshot is attached.
The thing that I'm confused about is that it says "As before, aij is the cosine of the angle between x′i and xj ." This is from rotating the coordinates, so x' is the new coordinates whilst x is the old one. And then it defined aij as the partial derivatives of the x'i wrt xj...Now I don't know how the cosine of the angle between x'i and xj is equal to the partial derivative. Can anyone explain this to me? Thank you in advanced!

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2. Jul 12, 2016

### Oz123

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3. Jul 12, 2016

### pasmith

If $$x'_i = \sum_j a_{ij} x_j$$ then $$\frac{\partial x'_i}{\partial x_k} = \sum_j a_{ij} \frac{\partial x_j}{\partial x_k} = a_{ik}$$ since $$\frac{\partial x_j}{\partial x_k} = \delta_{jk}$$.

4. Jul 12, 2016

### Oz123

Thank you very much!!!