已知a+b=5 ab=3 求代数式a三次方b-2a二次方吧

(a+b)^2=a^2+2ab+b^2=25
a^2+b^2=25-2ab=19
(a-b)^2=a^2+b^2-2ab=13
a-b=±√13
a+b=5
a=(5±√13)/2
a^3b-2a^2
=a^2(ab-2)
=a^2*1
=[(5±√13)/2]^2
=(38±10√13)/4
=(19±5√13)/2