B3:\(\Rightarrow90.10^n-10^n.10^2+10^n.10-20\Rightarrow10^n.\left(90-10^2\right)+10^n.10-20\)
\(\Rightarrow10^n.\left(90-100\right)+10^n.10-20\Rightarrow-10.10^n+10^n.10-20\Rightarrow-20\)
\(A=-\left(x^2-x+5\right)=-\left(x^2-2.\frac{1}{2}x+\frac{1}{4}+\frac{19}{4}\right)=-\left[\left(x-\frac{1}{2}\right)^2+\frac{19}{4}\right]\)
\(=-\left(x-\frac{1}{2}\right)^2-\frac{19}{4}\le-\frac{19}{4}\)
Vậy \(A_{min}=-\frac{19}{4}\Leftrightarrow x-\frac{1}{2}=0\Rightarrow x=\frac{1}{2}\)
B2: \(\Rightarrow16x^2-8x-\left(16x^2-8x+1\right)-13\Rightarrow16x^2-8x-16x^2+8x-1-13\Rightarrow-14\)
B4: \(\Rightarrow3\left(x^2+\frac{2}{3}x+\frac{28}{3}\right)\Rightarrow3\left(x^2+2.\frac{1}{3}.x+\frac{1}{9}+\frac{83}{9}\right)\Rightarrow3\left[\left(x+\frac{1}{3}\right)^2+\frac{83}{9}\right]\)
\(\Rightarrow3\left(x+\frac{1}{3}\right)^2+\frac{83}{3}\)
Vậy \(Min=\frac{83}{3}\Leftrightarrow x+\frac{1}{3}=0\Rightarrow x=-\frac{1}{3}\)
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