\(a,=x^2+x^3+4x+3x+3x^2+12=x^3+4x^2+7x+12\\ b,A=\left(5x+1\right)^2\\ c,=\left(3-x\right)\left(3+x\right)\\ d,\Leftrightarrow\left(5x-1\right)\left(x-20\right)=0\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{1}{5}\\x=20\end{matrix}\right.\\ e,A=\left(5x+1\right)^2-9=\left(5x+1-3\right)\left(5x+1+3\right)\\ A=\left(5x-2\right)\left(5x+4\right)\)
a) \(=x^2+x^3+4x+3x+3x^2+12=x^3+4x^2+7x+12\)
b) \(=\left(5x+1\right)^2\)
c) \(=\left(3-x\right)\left(3+x\right)\)
d) \(\Leftrightarrow5x\left(x-20\right)-\left(x-20\right)=0\\ \Leftrightarrow\left(x-20\right)\left(5x-1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=20\\x=\dfrac{1}{5}\end{matrix}\right.\)
e) \(=\left(25x^2+10x+1\right)-9=\left(5x+1\right)^2-3^2=\left(5x+1-3\right)\left(5x+1+3\right)=\left(5x-2\right)\left(5x+4\right)\)
