设两根是x1,x2,两根都大于1,那么有：b^2-4ac>=0 (i)（x1-1)(x2-1)>0 (ii)x1+x2>2 (iii)又x1x2=c/a>1>0=>c>0(ii)=>x1x2-(x1+x2)+1>0=>c/a+b/a+1>0=>a+b+c>a=>b+c>0=>b>-c(iii)=>-b/a>2=>b