[tex]\begin{gathered} x^2-25+(x^2-1) \\ =x^2-25+x^2\text{-1} \\ =2x^2-\text{2}6 \end{gathered}[/tex]
For problem 6:
[tex]\begin{gathered} (x-5)(x+5) \\ =x^2+5x-5x-25 \\ =x^2-25 \end{gathered}[/tex]
For problem 7:
[tex]\begin{gathered} (x-1)(x+1) \\ =x^2-x+x-1 \\ =x^2-1 \end{gathered}[/tex]
