N[...] 可能误差很大

hbghlyj

用N[...]得出-0.175781

1. Discriminant[-l-2 l^2-l^3-6 x+12 l x+16 l^2 x-2 l^3 x+24 x^2-36 l x^2-8 l^2 x^2-48 x^3+16 l x^3+48 x^4/.x->Root[5616 + 31104 # - 27000 #^2 - 26712 #^3 + 25119 #^4 - 5346 #^5 + 143 #^6 &, 4],l]//N

复制代码

用FullSimplify得出0

1. Discriminant[-l-2 l^2-l^3-6 x+12 l x+16 l^2 x-2 l^3 x+24 x^2-36 l x^2-8 l^2 x^2-48 x^3+16 l x^3+48 x^4/. x -> Root[5616 + 31104 # - 27000 #^2 - 26712 #^3 + 25119 #^4 - 5346 #^5 + 143 #^6 &, 4],l]//FullSimplify

复制代码