PQRS is a parallelogram and `oversetrarr(QS)` and `oversetrarr(PR)` are its diagonals .Show that
`vec(PR)+ vec(QS)-2vec(QR)=0`

Explanation

To prove that \( \vec{PR} + \vec{QS} - 2\vec{QR} = 0 \) for the parallelogram PQRS, we will follow these steps:

### Step 1: Understand the vectors in the parallelogram
In a parallelogram PQRS, we have:
- \( \vec{PQ} \) is one side,
- \( \vec{QR} \) is the adjacent side,
- \( \vec{PR} \) is the diagonal from point P to point R,
- \( \vec{QS} \) is the diagonal from point Q to point S.
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