\(A=4x-x^2+3\)
\(=-\left(-4x+x^2-3\right)\)
\(=-\left(x^2-4x-3\right)\)
\(=-\left(x^2-2.x.2+2^2-7\right)\)
\(=-\left[\left(x-2\right)^2-7\right]\)
\(=-\left(x-2\right)^2+7\le7\)
Dấu = xảy ra khi:
\(\left(x-2\right)^2=0\)
\(\Rightarrow x-2=0\)
\(\Rightarrow x=2\)