Which one of the following equations is a correct identity

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Which one of the following equations is a correct identity for arbitrary 3 × 3 real matrices P, Q and R?

1. P(Q + R) = PQ + RP
2. (P – Q)² = P² – 2PQ + Q²
3. det (P + Q) = detP + detQ
4. (P + Q)² = P² + PQ + QP + Q²

Correct Option: D

This is a standard relation.
(P + Q)² = P² + PQ + QP + Q²