x1+x2=-b
x1*x2=c
x3+x4=-b^2
x3*x4=20'
x2-x1=x3-x4
b^2-4c=b^4-80
-b+b^2=x2-x3+x1-x4=6
b=-2,或b=3 (b^4-80>0,所以-2舍去）
b=3,c=2