Evaluate `|[1,x, y],[1,x+y, y],[1,x,x+y]|`

Explanation

To evaluate the determinant \( | \begin{bmatrix} 1 & x & y \\ 1 & x+y & y \\ 1 & x & x+y \end{bmatrix} | \), we will follow these steps:

### Step 1: Write the Determinant
We start with the determinant:
\[ D = \begin{vmatrix} 1 & x & y \\ 1 & x+y & y \\ 1 & x & x+y \end{vmatrix} \]

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