If the vertices A, B, C of a triangle ABC are `(1, 2, 3), (1, 0, 0), (0, 1, 2)`, respectively, then find `/_A B C`. [`/_A B C`is the angle between the vectors ` vec( B A)`and ` vec (B C)`.

Explanation

To find the angle \( \angle ABC \) between the vectors \( \vec{BA} \) and \( \vec{BC} \), we will follow these steps:

### Step 1: Define the coordinates of the points
The coordinates of the vertices of triangle ABC are:
- \( A(1, 2, 3) \)
- \( B(1, 0, 0) \)
- \( C(0, 1, 2) \)

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