设f(x)=x^2-2x-5则f(3)=-2 f(4)=3f(3.5)=0.25f[(3+3.5)/2]=f(3.25)=-0.9375f[(3.25+3.5)/2]=f(3.375)=-0.359375f[(3.375+3.5)/2]=f(3.4375)=-0.05859375f(3.45)=0.003085937所以3.4375