\[x^{4} - 26x^{2} + 25 =\]
\[= x^{4} - 25x^{2} - x^{2} + 25 =\]
\[= x^{2}\left( x^{2} - 25 \right) - \left( x^{2} - 25 \right) =\]
\[= \left( x^{2} - 1 \right)\left( x^{2} - 25 \right) =\]
\[= (x - 1)(x + 1)(x - 5)(x + 5)\]