a) \(2x^2+4x+3=2.\left(x^2+2x+1\right)+1\)
\(=2.\left(x+1\right)^2+1\ge0\)
Dấu "=" xảy ra \(\Leftrightarrow x+1=0\Leftrightarrow x=-1\)
b) \(x^2+5x-3=x^2+2.x.\frac{5}{2}+\left(\frac{5}{2}\right)^2-\frac{27}{4}\)
\(=\left(x+\frac{5}{2}\right)^2-\frac{27}{4}\ge-\frac{27}{4}\)
Dấu "=" xảy ra \(\Leftrightarrow x+\frac{5}{2}=0\Leftrightarrow x=-\frac{5}{2}\)