a: \(=x^2+6x+9+1=\left(x+3\right)^2+1>=1\)
Dấu '=' xảy ra khi x=-3
b: \(=2\left(x^2+\dfrac{3}{2}x+\dfrac{5}{2}\right)\)
\(=2\left(x^2+2\cdot x\cdot\dfrac{3}{4}+\dfrac{9}{16}+\dfrac{31}{16}\right)\)
\(=2\left(x+\dfrac{3}{4}\right)^2+\dfrac{31}{8}>=\dfrac{31}{8}\)
Dấu '=' xảy ra khi x=-3/4
c: \(=x^2-6x+9+y^2+8y+16-20\)
\(=\left(x-3\right)^2+\left(y+4\right)^2-20>=-20\)
Dấu '=' xảy ra khi x=3 và y=-4