+) ta có : \(E=3x^2-6x+15=3\left(x^2-2x+1\right)+12\)
\(=3\left(x-1\right)^2+12\ge12\) \(\Rightarrow E_{min}=12\) khi \(x=1\)
+) ta có : \(F=5x^2+6x-12=5\left(x^2+\dfrac{6}{5}x+\dfrac{9}{25}\right)-\dfrac{69}{5}\)
\(=5\left(x+\dfrac{3}{5}\right)^2-\dfrac{69}{5}\ge\dfrac{-69}{5}\) \(\Rightarrow F_{min}=-\dfrac{69}{5}\) khi \(x=\dfrac{-3}{5}\)
+) ta có : \(G=4x^2-4x+25=4\left(x^2-x+\dfrac{1}{4}\right)+24\)
\(=4\left(x-\dfrac{1}{2}\right)^2+24\ge24\) \(\Rightarrow G_{min}=24\) khi \(x=\dfrac{1}{2}\)
+) ta có : \(H=9x^2+6x^2+4=15x^2+4\ge4\)
\(\Rightarrow H_{min}=4\) khi \(x=0\)
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Tìm GTNN
E=3x^2-6x+15
F= 5x^2+6x-12
G=4x^2-4x+25
H=9x^2+6x^2+4
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