D=5|3x+7|-10
Ta thấy:\(\left|3x+7\right|\ge0\)
\(\Rightarrow5.\left|3x+7\right|\ge5.0=0\)
\(\Rightarrow5.\left|3x+7\right|-10\ge0-10=-10\)
\(\Rightarrow D\ge-10\)
Dấu = khi 5|3x+7|-10=-10 =>5|3x+7|=0
=>|3x+7|=0 =>3x=-7<=>x=-7/3
Vậy Dmin--10 <=>x=-7/3