a, \(A=4x^2-28x+49=\left(2x\right)^2+2.2x.7+7^2=\left(2x+7\right)^2\)
Với x = 4 \(\Rightarrow A=\left(2.4+7\right)^2=15^2=225\)
b, \(B=x^3-9x^2+27x-27\)
\(=\left(x^3-3x^2\right)-\left(6x^2-18x\right)+\left(9x-27\right)\)
\(=x^2\left(x-3\right)-6x\left(x-3\right)+9\left(x-3\right)\)
\(=\left(x-3\right)\left(x^2-6x+9\right)\)
\(=\left(x-3\right)\left(x^2-2.x.3+3^2\right)\)
\(=\left(x-3\right)\left(x-3\right)^2=\left(x-3\right)^3\)
Với x = 5 \(\Rightarrow B=\left(5-3\right)^3=2^3=8\)