$\text{ If the approximate value of }(1.999{)}^6\text{ is }$

$\begin{array}{r}\text{ We write }1.999=2-0.001\\ \begin{array}{l}x=0\text{ and }\mathrm{\Delta }x=-0.001\\ f\left(1.999\right)-f\left(2\right)=6\left(2^5\right)\times (-0.001)\\ \Rightarrow f\left(1.999\right)=63.81\end{array}\end{array}$