4≦t²-3t≦40可分解为4≦t²-3t和t²-3t≦40先计算4≦t²-3t ①t²-3t≧4t²-3t-4≧0(t+1)(t-4)≧0∴t≧-1或t≧4要使①恒成立，此时t≧4再计算t²-3t≦40 ②t²-3t-40≦0(t-8)(t+5)≦0∴t≦8或t≦-5要使②恒成立，此时t≦-5综上，t≧4或t≦-5
－5≤t≤－1或4≤t≤8
