0.8^3=0.512
设（0.8-x)^3=0.43
0.8^3-3*0.8^2*x≈0.43
x≈（0.8^3-0.43)/ (3*0.8^2)≈0.0427
0.8-x≈0.757
0.43的立方根是0.757