\(f\left(x\right)=-x^2-5x-10=-\left(x^2+2x\cdot\dfrac{5}{2}+\left(\dfrac{5}{2}\right)^2\right)+\left(\dfrac{5}{2}\right)^2-10\)
\(f\left(x\right)=-\left(x+\dfrac{5}{2}\right)^2-\dfrac{15}{4}\)
ta thay \(-\left(x+\dfrac{5}{2}\right)^2\le0\)
\(\Rightarrow-\left(x+\dfrac{5}{2}\right)^2-\dfrac{15}{4}\le-\dfrac{15}{4}\)
\(\Rightarrow-\left(x+\dfrac{5}{2}\right)^2-\dfrac{15}{4}< 0\)
suy ra da thuc f(x) vo nghiem
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