Question
Factorise: $y^2-21 y+90$
✓
Answer
$y^2-21 y+90$
$=y^2-15 y-6 y+90$
$\{90 = (-15) × (-6), -21 = -15 - 6\}$
$= y(y - 15) - 6(y - 15) $
$= (y - 15)(y - 6)$
$=y^2-15 y-6 y+90$
$\{90 = (-15) × (-6), -21 = -15 - 6\}$
$= y(y - 15) - 6(y - 15) $
$= (y - 15)(y - 6)$
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