\[\begin{array}{l}
Bai\,\,40 - sgk\,\,trang\,\,\,19\\
a)\,\,15.91,5 + 150.0,85\\
= 15.91,5 + 15.8,5\\
= 15\left( {91,5 + 9,5} \right) = 15.100 = 1500.\\
b)\,\,x\left( {x - 1} \right) - y\left( {1 - x} \right)\\
= x\left( {x - 1} \right) + \left( {x - 1} \right)y\\
= \left( {x - 1} \right)\left( {x + y} \right).\\
Thay\,\,\,\,x = 2001;\,\,\,y = 1999\,\,\,vao\,\,\,bieu\,\,thuc\,\,ta\,\,duoc:\\
\left( {2001 - 1} \right)\left( {2001 + 1999} \right) = 2000.4000 = 8000000.
\end{array}\]
