Factorise $pq - rs + ps - qr$.

Factorise $pq - rs + ps - qr$.
  1. $(p - r)(q + s)$
  2. $(p + r)(q + s)$
  3. $(p - r)(q - s)$
  4. $(p + r)(q - s)$

Solution

Rearrange: $pq + ps - qr - rs = p(q+s) - r(q+s) = (p-r)(q+s)$.

Asked in: IMO

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