Giải:
a) \(C=4x^2-8x+100\)
\(\Leftrightarrow C=4x^2-8x+4+96\)
\(\Leftrightarrow C=\left(4x^2-8x+4\right)+96\)
\(\Leftrightarrow C=\left(2x-2\right)^2+96\ge96;\forall x\)
\(\Leftrightarrow C_{Min}=96\)
\("="\Leftrightarrow2x-2=0\Leftrightarrow x=1\)
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b) \(D=\left|x\right|+5+\left(-155-\dfrac{2}{5}\right)\)
\(\Leftrightarrow D=\left|x\right|+5-155-\dfrac{2}{5}\)
\(\Leftrightarrow D=\left|x\right|-150-\dfrac{2}{5}\)
\(\Leftrightarrow D=\left|x\right|-150,4\ge-150,4;\forall x\)
\(\Leftrightarrow D_{Min}=-150,4\)
\("="\Leftrightarrow\left|x\right|=0\Leftrightarrow x=0\)
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