# How do you multiply (sqrt10 - 9)^2 and write the product in simplest form?

You can write (considering that ${\left(a - b\right)}^{2} = {a}^{2} - 2 a b + {b}^{2}$):
$\left(\sqrt{10} - 9\right) \cdot \left(\sqrt{10} - 9\right) = {\left(\sqrt{10}\right)}^{2} - 2 \cdot 9 \cdot \sqrt{10} + {9}^{2} =$
$= 10 - 18 \sqrt{10} + 81 =$
$= 91 - 18 \sqrt{10} =$