a/ \(y=\frac{1}{4}.4x\left(1-x\right)\left(1-x\right)\left(1-x\right)\left(1-x\right)\)
\(y\le\frac{1}{4}\left(\frac{4x+1-x+1-x+1-x+1-x}{5}\right)^5=\frac{4^4}{5^5}\)
Dấu "=" xảy ra khi \(4x=1-x\Leftrightarrow x=\frac{1}{5}\)
b/ \(y=\frac{x-3}{3}+\frac{48}{x-3}\ge2\sqrt{\frac{48\left(x-3\right)}{3\left(x-3\right)}}=8\)
Dấu "=" xảy ra khi \(\left(x-3\right)^2=144\Rightarrow x=15\)