Tìm gtnn a) x² +6xy +16y² +28y +30 b) 5x² +9y² -12 +24x -48y+82
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a)Đặt \(A=3x^2-x+1\)
\(A=3\left(x^2-2.\frac{1}{6}x+\frac{1}{36}\right)+\frac{11}{12}\)
\(A=3\left(x-\frac{1}{6}\right)^2+\frac{11}{12}\)
Vì \(3\left(x-\frac{1}{6}\right)^2\ge0\Rightarrow3\left(x-\frac{1}{6}\right)^2+\frac{11}{12}\ge\frac{11}{12}\)
Dấu = xảy ra khi \(x-\frac{1}{6}=0\Rightarrow x=\frac{1}{6}\)
Vậy Min A = \(\frac{11}{12}\) khi x=1/6
b)Tương tụ
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\(4x^2+9y^2+64-12xy-48y+32x+x^2-8x+16+2\)
\(=\left(2x-3y+8\right)^2+\left(x-4\right)^2+2\ge2\)
Dấu "=" xảy ra \(\Leftrightarrow\)x=4 và y=\(\frac{16}{3}\)
Vậy MINP=2 <=> x=4;y=16/3
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\(s=\left(2x-3y+8\right)^2+\left(x-4\right)^2+2014-16-64\)
\(S_{min}=2014-16-164\)
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