Factorise $25a^{2} - 49b^{2}$.

Factorise $25a^{2} - 49b^{2}$.
  1. $(5a - 7b)(5a + 7b)$
  2. $(5a - 7b)^{2}$
  3. $(25a - 49b)(a + b)$
  4. $5(a-b)(5a+7b)$

Solution

$25a^{2} - 49b^{2} = (5a)^{2} - (7b)^{2} = (5a-7b)(5a+7b)$.

Asked in: IMO

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