If the vertices A,B,C of a triangle ABC are (1,2,3),(-1,0,0) ,(0,1,2) , respectively, then find `angleABC`.

Explanation

To find the angle \( \angle ABC \) in triangle ABC with vertices \( A(1, 2, 3) \), \( B(-1, 0, 0) \), and \( C(0, 1, 2) \), we will follow these steps:

### Step 1: Find the position vectors of points A, B, and C
The position vectors can be represented as:
- \( \vec{A} = \begin{pmatrix} 1 \\ 2 \\ 3 \end{pmatrix} \)
- \( \vec{B} = \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \)
- \( \vec{C} = \begin{pmatrix} 0 \\ 1 \\ 2 \end{pmatrix} \)

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