用反证法:假设|f(1)|,|f(2)|,|f(3)|,全小于1/2;即 |a+b+1|<1/2,...-1/2<a+b+1<1/2.... ...-1/2<-(a+b+1)<1/2..........(1) |2a+b+4|<1/2,...-1/2<2a+b+4<1/2..........(2) -1/2<-(2a+b+4)<1/2......(3) |3a+b+9|<1/2,....-1/2<3a+b+9<1/2.........(4) (1)+(2).得.-1<a+3<1.......-4<a<-2...(5) (3)+(4)..得..-1<a+5<1......-6<a<-4...(6) 所以(5)与(6)矛盾,...
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