Y^2 21y+90 =y^2 15y 6y+90 ∵ 90=( 15)× ( 6) amp; 21= 15 6 amp; =y(y 15) 6(y 15) =(y 15)(y 6) ...

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Factorisation by Regrouping Terms

Factorise: y2...

Question

Factorise:
$y^{2} - 21 y + 90$

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Solution

$y^{2} - 21 y + 90 = y^{2} - 15 y - 6 y + 90 {\begin{matrix} ∵ 90 = (- 15) \times (- 6) - 21 = - 15 - 6 \end{matrix}} = y (y - 15) - 6 (y - 15) = (y - 15) (y - 6)$

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