The expression x21−x2 can not be simplified, therefore, it can be expressed as :
x21−x4
or
x2(1−x2)(1+x2)
The expression x21−x2 combines two different terms that involve the variable x.
These terms cannot be simplified together algebraically since one is rational (a fraction) and the other is polynomial. Therefore, the expression remains as is:
x21−x2
When expressing everything with a common denominator:
x21−x2x4=x21−x4
Since 1−x4 can be factored as:
1−x4=(1−x2)(1+x2)
The expression then simplifies to:
x2(1−x2)(1+x2)
So the final simplified form is:
x2(1−x2)(1+x2)
In conclusion :
x21−x2=x21−x4=x2(1−x2)(1+x2)