71/ Pt có 2 nghiệm trái dấu
\(→k^2-9<0\\↔(k-3)(k+3)<0\\↔\left[\begin{array}{1}\begin{cases}k-3>0\\k+3>0\end{cases}\\\begin{cases}k-3<0\\k+3<0\end{cases}\end{array}\right.\\↔\left[\begin{array}{1}\begin{cases}k>3\\k>-3\end{cases}\\\begin{cases}k<3\\k<-3\end{cases}\end{array}\right.\\↔\left[\begin{array}{1}k>3\\k<-3\end{array}\right.\)
72/ Theo Vi-ét:
\(\begin{cases}x_1+x_2=\dfrac{-b}{a}\\x_1x_2=\dfrac{c}{a}\end{cases}\)
\(\dfrac{1}{x_1}+\dfrac{1}{x_2}\\=\dfrac{x_1+x_2}{x_1x_2}\\=\dfrac{\dfrac{-b}{a}}{\dfrac{c}{a}}\\=-\dfrac{b}{c}\)
73/ Pt có nghiệm
\(→Δ'=(-4)^2-(2a-1).6=16-12a+6=22-12a\ge 0\\↔a\le \dfrac{11}{6}\)
mà a lớn nhất, a nguyên
\(→a=1\)
