If the vectors `A ,B ,C` of a triangle `A B C` are `(1,2,3),(-1,0,0),(0,1,2),` respectively then find `/_A B Cdot`

Explanation

To find the angle \( \angle ABC \) in triangle \( ABC \) given the position vectors of the vertices \( A, B, C \), we can follow these steps:

### Step 1: Define the Position Vectors
The position vectors of the vertices are given as:
- \( \vec{A} = (1, 2, 3) \)
- \( \vec{B} = (-1, 0, 0) \)
- \( \vec{C} = (0, 1, 2) \)

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