a: \(C=-x^2-5x+5\)
\(=-x^2-5x-\dfrac{25}{4}+\dfrac{25}{4}+5\)
\(=-\left(x+\dfrac{5}{2}\right)^2+\dfrac{45}{4}< =\dfrac{45}{4}\forall x\)
Dấu '=' xảy ra khi \(x+\dfrac{5}{2}=0\)
=>\(x=-\dfrac{5}{2}\)
b: \(B=-x^2-4x+3\)
\(=-x^2-4x-4+7\)
\(=-\left(x+2\right)^2+7< =7\forall x\)
Dấu '=' xảy ra khi x+2=0
=>x=-2
c: \(C=-x^2-5x+8\)
\(=-x^2-5x-\dfrac{25}{4}+\dfrac{25}{4}+8\)
\(=-\left(x^2+5x+\dfrac{25}{4}\right)+\dfrac{57}{4}\)
\(=-\left(x+\dfrac{5}{2}\right)^2+\dfrac{57}{4}< =\dfrac{57}{4}\forall x\)
Dấu '=' xảy ra khi \(x+\dfrac{5}{2}=0\)
=>\(x=-\dfrac{5}{2}\)
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