\[\boxed{\mathbf{Решаем}\mathbf{\ }\mathbf{устно}\mathbf{\ }\mathbf{параграф}\mathbf{\ }\mathbf{38}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\boxed{\mathbf{1.}}\]
\[- 2,5 \cdot 2 \cdot ( - 10) =\]
\[= - 5 \cdot ( - 10) = 50.\]
\[\boxed{\mathbf{2.}}\]
\[- 2,5 \cdot \left( 2 \cdot ( - 10) \right) =\]
\[= - 2,5 \cdot ( - 20) = 50.\]
\[\boxed{\mathbf{3.}}\]
\[x = 4 \Longrightarrow - 1,5 \cdot 4 = - 6\]
\[x = - 100\ \Longrightarrow - 1,5 \cdot ( - 100) =\]
\[= 150\]
\[x = 0 \Longrightarrow - 1,5 \cdot 0 = 0\]
\[x = - \frac{1}{3}\ \Longrightarrow - 1,5 \cdot \left( - \frac{1}{3} \right) =\]
\[= \frac{15 \cdot 1}{10 \cdot 3} = \frac{1}{2}\]
\[x = - 1\ \Longrightarrow - 1,5 \cdot ( - 1) = 1,5\]
\[x = 0,2 \Longrightarrow - 1,5 \cdot 0,2 = - 0,3\]
\[\boxed{\mathbf{4.}}\]
\[1) - 3a < 0 \Longrightarrow a -\]
\[положительное.\]
\[2)\ \frac{1}{6}a < 0 \Longrightarrow a -\]
\[отрицательное.\]
\[3) - 0,7a > 0 \Longrightarrow a -\]
\[отрицательное.\]
\[\boxed{\mathbf{5.}}\]
\[1 - 3 + 5 - 7 + 9 - 11 + \ldots +\]
\[+ 97 - 99 = - 2 + ( - 2) +\]
\[+ ( - 2) + .. + ( - 2) =\]
\[= - 2 \cdot 25 = - 50.\]