六种极限 lim f(x) lim f(x); x→+ x→>x0 lim f(x); im f(e); x→x lim f(); lim f(r); x→ x→x六种极限 lim f (x); x→+  lim f (x); x→−  lim f (x); x→ lim ( ); 0 f x x→x lim ( ); 0 f x x x → + lim ( ); 0 f x x x → −