\(\left(1+x^2\right)^2-4x\left(1-x^2\right)\)
\(\Leftrightarrow\left(1+x^2\right)^2+4x\left(1+x^2\right)\)
\(\Leftrightarrow\left(1+x^2\right)\times\left[\left(1+x^2\right)+4\right]\)
( 1+x2 )2 -4x( 1- x2 )
=x4+2x2+1-4x+4x3
=x3+2x2-x+2x3+4x2-2x-x2-2x+1
=x(x2+2x-1)+2x(x2+2x-1)-(x2+2x-1)
=(x2+2x-1)(x2+2x-1)
=(x2+2x-1)2