Câu 1:
$(P): y = ax^2 + 4x - b\quad (a \ne 0)$
$(P)$ có đỉnh $I(-1;-5)$
$\to \begin{cases}-\dfrac{4}{2a} = -1\\-\dfrac{4^2 + 4ab}{4a} = -5\end{cases}$
$\to \begin{cases}a = 2\\16 + 4ab = 20a\end{cases}$
$\to \begin{cases}a = 2\\b = 3\end{cases}$
Vậy $(a;b)= (2;3)$
Câu 2:
$\begin{array}{l}\quad |3-x| = |2x-5|\\ \Leftrightarrow \left[\begin{array}{l}3- x = 2x - 5\\3 - x = 5 - 2x\end{array}\right.\\ \Leftrightarrow \left[\begin{array}{l}3x = 8\\x = 2\end{array}\right.\\ \Leftrightarrow \left[\begin{array}{l}x = \dfrac83\\x = 2\end{array}\right.\\ \Rightarrow x_1 + x_2 = \dfrac83 + 2 = \dfrac{14}{3} \end{array}$
