设一元四次方程为ax^4+bx^3+cx^2+dx+e=0
x1+ x2+ x3+ x4= -b/a
x1x2 +x1x3+ x1x4+ x 2 x3 + x2x4+ x3 x4=c/a
x1x2x3 +x1x2x4+ x1 x3 x4+ x2 x3 x4= -d/a
x1 x2 x3 x4=e/a