a. Khi m = 3, ta có pt:
\(x^2-10x+16=0\)
\(\Leftrightarrow x^2-8x-2x+16=0\)
\(\Leftrightarrow x\left(x-8\right)-2\left(x-8\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(x-8\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=8\end{matrix}\right.\)
b. \(\Delta'=b'^2-ac=\left(m+2\right)^2-m^2-3m+2=m+6\)
Để pt có nghiệm phân biệt => \(\Delta'>0\Rightarrow m+6>0\Rightarrow m>-6\)
Hệ thức vi-et: \(\left\{{}\begin{matrix}x_1+x_2=2m+4\\x_1.x_2=m^2+3m-2\end{matrix}\right.\)
\(x_1^2+x_2=2m^2+10m+20\)
\(A=2018+3\left(m^2+3m-2\right)-\left(2m^2+10m+20\right)=m^2-m+1992=\left(m-\frac{1}{2}\right)^2+\frac{7967}{4}\ge\frac{7967}{4}\)
Vậy Min A = \(\frac{7967}{4}\Leftrightarrow m=\frac{1}{2}\)