Question

Factorise the following $y^2-18y+81$

Answer 1

$y=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$

Here $a=1,b=-18,c=81$

$y=\frac{18\pm \sqrt{(-18{)}^2-4\times 1\times 81}}{2\times 1}$

$y=\frac{18\pm \sqrt{0}}{2}$

$y=\frac{18}{2}$

$y=9$

$y^2-18y+81=(y-9)(y-9)=(y-9{)}^2$