f(x)=x^3-4 a=1 ,b =2 f(1)*f(2) < 0
(a+b)/2=1.5 ,f(1.5) = 3.375 -4 = - 0.625
取 a =1.5 ,b= 2 (a+b)/2=1.75 ,f(1.75) =5.36 -4 > 0
a =1.5 ,b= 1.75 (a+b)/2=1.63 ,f(1.63) =4.33 -4 > 0
a =1.5 ,b= 1.63 (a+b)/2=1.57 ,f(1.57) =3.87 -4