Factorise $25 - 30y + 9y^{2}$.

Factorise $25 - 30y + 9y^{2}$.
  1. $(5 - 3y)^{2}$
  2. $(5 + 3y)^{2}$
  3. $(5 - 3y)(5 + 3y)$
  4. $(3y - 5)(3y + 5)$

Solution

$25-30y+9y^{2} = 5^{2} - 2 \cdot 5 \cdot 3y + (3y)^{2} = (5-3y)^{2}$.

Asked in: IMO

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