\(A=\frac{3}{4x^2-4x+5}\)
\(=\frac{3}{4x^2-4x+1+4}\)
\(=\frac{3}{\left(2x-1\right)^2+4}\)
\(\left(2x-1\right)^2\ge0\)
\(\Rightarrow\left(2x-1\right)^2+4\ge4\)
\(\Rightarrow\frac{3}{\left(2x-1\right)^2+4}\le\frac{3}{4}\)
\(MaxA=\frac{3}{4}\Leftrightarrow x=\frac{1}{2}\)
Đặt \(A=\frac{3}{4x^2-4x+5}\)
Biến đổi : \(4x^2-4x+5\)
\(=\left[\left(2x\right)^2-2.2x.1+1^2\right]+4\)
\(=\left(2x-1\right)^2+4\)
Ta có : \(\left(2x-1\right)^2\ge0\)
\(\Rightarrow\left(2x-1\right)^2+4\ge4\)
\(\Rightarrow\frac{3}{\left(2x-1\right)^2+4}\le\frac{3}{4}\)
\(\Rightarrow A\le\frac{3}{4}\)
Dấu " = " xảy ra khi và chỉ khi \(2x-1=0\)
\(2x=1\)
\(x=\frac{1}{2}\)
Vậy \(Max_A=\frac{3}{4}\Leftrightarrow x=\frac{1}{2}\)
