\(D=5x^2+5x+1\)
\(=5\left(x^2+x+\dfrac{1}{5}\right)\)
\(=5\left(x^2+x+\dfrac{1}{4}-\dfrac{1}{20}\right)\)
\(=5\left(x^2+x+\dfrac{1}{4}\right)-\dfrac{1}{4}=5\left(x+\dfrac{1}{2}\right)^2-\dfrac{1}{4}\ge-\dfrac{1}{4}\forall x\in R\)
- Vậy \(MinD=-\dfrac{1}{4}\), đạt tại \(x=-\dfrac{1}{2}\)