Đáp án:
a) t=4
Giải thích các bước giải:
\(\begin{array}{l}
a)V\left( 3 \right) = 12000000 - 1400000.3 = 7800000\\
V\left( t \right) = 6400000\\
\to 12000000 - 1400000.t = 6400000\\
\to t = 4\\
b)\left\{ \begin{array}{l}
y = 2x - 5\\
mx + 3\left( {2x - 5} \right) = 4
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 2x - 5\\
mx + 6x - 15 = 4
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{{19}}{{m + 6}}\\
y = 2.\dfrac{{19}}{{m + 6}} - 5 = \dfrac{{38 - 5m - 30}}{{m + 6}}
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{{19}}{{m + 6}}\\
y = \dfrac{{8 - 5m}}{{m + 6}}
\end{array} \right.\\
DK:m \ne - 6\\
Do:xy > 0\\
\to \dfrac{{19}}{{m + 6}}.\dfrac{{8 - 5m}}{{m + 6}} > 0\\
\to \dfrac{{19\left( {8 - 5m} \right)}}{{{{\left( {m + 6} \right)}^2}}} > 0\\
\to 8 - 5m > 0\left( {do:{{\left( {m + 6} \right)}^2} > 0\forall m \ne - 6} \right)\\
\to \dfrac{8}{5} > m;m \ne - 6
\end{array}\)
