$\begin{array}{l}a)\,\,A=|x|+5\\\text{- Ta có :}\\|x|\geq0\\\to|x|+5\geq5\\\text{- Dấu "=" xảy ra}\\ \Leftrightarrow |x|=0\\\Leftrightarrow x=0\\\text{- Vậy $A_\min=5 \Leftrightarrow x=0$}\\\,\\b)\,\,B=-|x+1|+50=50-|x+1|\\\text{- Ta có :}\\|x+1|\geq0\\\to50-|x+1|\leq50\\\text{- Dấu "=" xảy ra}\\\Leftrightarrow |x+1|=0\\\Leftrightarrow x+1=0\\\Leftrightarrow x=-1\\\text{- Vậy $B_\max =50\Leftrightarrow x=-1$}\\\,\\c)\,\,C=|x-3|-2021\\\text{- Ta có :}\\|x-3|\geq0\\\to |x-3|-2021\geq-2021\\\text{- Dấu "=" xảy ra }\\\Leftrightarrow |x-3|=0\\\Leftrightarrow x-3=0\\\Leftrightarrow x=3\\\text{- Vậy $C_\min = -2021\Leftrightarrow x=3$}\\\,\\d)\,\,\text{- Kiểm tra lại đề!} \end{array}$
