\(A=7,5-\left|2x+1,8\right|=-\left|2x+1,8\right|+7,5\)
+Có: \(-\left|2x+1,8\right|\le0với\forall x\)
\(\Rightarrow-\left|2x+1,8\right|+2\le2\)
\(\Leftrightarrow A\le2\)
+Dấu "=" xảy ra khi \(\left|2x+1,8\right|=0\Rightarrow x=-0,9\)
+Vậy \(A_{max}=2\) khi \(x=-0,9\)
\(B=-\left|10,5-3x\right|-15\)
+Có: \(-\left|10,5-3x\right|\le0với\forall x̸\)
\(\Rightarrow-\left|10,5-3x\right|-15\le-15\)
\(\Leftrightarrow B\le-15\)
+Dấu "=" xảy ra khi \(\left|10,5-3x\right|=0\Rightarrow x=3,5\)
+Vậy \(B_{max}=-15\) khi \(x=3,5\)
\(C=-\left|4x-5\right|-\left|3y+21\right|-15\)
+Có: \(-\left|4x-5\right|\le0với\forall x\)
\(-\left|3y+21\right|\le0với\forall y\)
\(\Rightarrow-\left|4x-5\right|-\left|3y+21\right|-15\le-15\)
\(\Leftrightarrow C\le-15\)
+Dấu "=" xảy ra khi \(\left|4x-5\right|=0\Rightarrow x=\frac{5}{4};\left|3y+21\right|=0\Rightarrow x=-7\)
+Vậy \(C_{max}=-15\) khi x=\(\frac{5}{4}\) y=\(-7\)
