\left(1+x^2\right)^2-4x\left(1-x^2\right)\\ =\left(1+x^2\right)^2-4x\left(1-x^2\right)+4x^2-4x^2\\ =\left(1-x^2-2x\right)^2-4x^2\\ =\left(1-x^2-2x+2x\right)\left(1-x^2-2x-2x\right)\\ =\left(1-x^2\right)\left(1-x^2-4x\right)\\ =\left(1-x\right)\left(1+x\right)\left(1-x^2-4x\right) |
\(\left(1+x^2\right)^2-4x\left(1-x^2\right)\)
\(=\left[\left(1+x^2\right)-4x^2\right]+4x\left(1-x^2\right)+4x^2\)
\(=\left(1+x^2-2x\right)\left(1+x^2+2x\right)+4x\left(1-x^2\right)+4x^2\)
\(=\left(1-x\right)^2\left(1+x\right)^2-4x\left(1-x^2\right)+4x^2\)
\(=\left(1-x^2\right)^2-4x\left(1-x^2\right)+4x^2\)
\(=\left(1-x^2-2x\right)^2\)