1. \(x^2+y^2+2x-4y+5\)
\(=\left(x^2+2x+1\right)+\left(y^2-4y+4\right)\)
\(=\left(x+1\right)^2+\left(y-2\right)^2\)
2. \(x^2+4y^2-x-4y+\dfrac{5}{4}\)
\(=\left(x^2-x+\dfrac{1}{4}\right)+\left(4y^2-4y+1\right)\)
\(=\left(x-\dfrac{1}{2}\right)^2+\left(2y-1\right)^2\)
3. \(x^2+4x+20\)
\(=x^2+4x+4+16\)
\(=\left(x+2\right)^2+16\ge16\)
Vậy \(x^2+4x+20\) dương.