Compute $103 \times 105$ using identities.

Compute $103 \times 105$ using identities.
  1. $10815$
  2. $10800$
  3. $10915$
  4. $10515$

Solution

$(100+3)(100+5) = 100^{2} + 8(100) + 15 = 10000 + 800 + 15 = 10815$.

Asked in: IMO

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