dn10, you have a very hard road of learning ahead of you if you truly want to understand this material.
You should pay more attention to basic definitions.
Of course, if you could care less then forget it.
$\displaystyle t^{4}-1=\left( t^{2}+1 \right)\left( t^{2}-1 \right)>0,$ this last 'cause logarithm is defined for numbers greater than zero. Hence, it remains to solve $\displaystyle t^2-1>0$ which gives the expected answer.