Question

Factorise:
$x^2-23x+42$

Answer 1

$x^2-23x+42=x^2-2x-21x+42\left\{\begin{array}{cc}\because 42=(-2)\times (-21)& \\ -23=-2-21& \end{array}\right\}=x(x-2)-21(x-2)=(x-2)(x-21)Ans.$