解説：
$$ \begin{eqnarray}
(x+\color{red}{a})(x+\color{blue}{b}) &=& x^2+(\color{red}{a}+\color{blue}{b})x+ \color{red}{a} \times \color{blue}{b} \\
\end{eqnarray} $$
$$ \begin{eqnarray}
x(x-1)(x-2) &=& \\
&& 上記の公式で、\color{red}{a=-1}、\color{blue}{b=-2}となるので、\\
x(x-1)(x-2) &=& x \times (x^2+(\color{red}{-1}\color{blue}{-2})x+(\color{red}{-1})\times(\color{blue}{-2})) \\
&=& x\times(x^2-3x+2) \\
&=& x^3-3x^2+2x\\
\end{eqnarray} $$
以上。