|f(x)| < a (a>0): equivalent to -a < f(x) < a (conjunction). |f(x)| > a (a>0): equivalent to f(x) < -a OR f(x) > a (disjunction). |f(x)| <= a: includes endpoints. |f(x)| >= a: includes endpoints. Special cases: |f(x)| < 0 has no solution; |f(x)| <= 0 iff f(x) = 0; |f(x)| >= 0 is always true.