Bài 4:
\(a,A=x^2+5\ge5\\ A_{min}=5\Leftrightarrow x=0\\ b,B=\left|x+1\right|+3\ge3\\ B_{min}=3\Leftrightarrow x+1=0\Leftrightarrow x=-1\)
Bài 5:
\(a,A=-x^2+3\le3\\ A_{max}=3\Leftrightarrow x=0\\ b,B=7-\left|x+5\right|\le7\\ B_{max}=7\Leftrightarrow x+5=0\Leftrightarrow x=-5\\ c,C=4,5-\left|1,5x-7,5\right|\le4,5\\ C_{max}=4,5\Leftrightarrow1,5x=7,5\Leftrightarrow x=5\\ d,Vì.3+\left|2x-1\right|\ge3\\ \Leftrightarrow D=\dfrac{2}{3+\left|2x-1\right|}\le\dfrac{2}{3}\\ D_{max}=\dfrac{2}{3}\Leftrightarrow2x=1\Leftrightarrow x=\dfrac{1}{2}\)
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