a: \(=\left(x+2\right)^2-4y^2=\left(x+2+2y\right)\left(x+2-2y\right)\)
b: \(=x^3\left(x+1\right)+x\left(x+1\right)\)
\(=x\left(x+1\right)\left(x^2+1\right)\)
d: \(=\left(x-2y\right)^2-\left(z-2t\right)^2\)
\(=\left(x-2y-z+2t\right)\left(x-2y+z-2t\right)\)