This is how I did the 1st one

来源: 于 2016-11-11 10:31:11 [旧帖] [给我悄悄话] 本文已被阅读：次

15x^5+17x^4+x^3-x^2=0

X^2(15x^3+17x-1)=0

Since 1/5 is a root:

X^2(15X^3+17X-1)

= X^2(X-1/5)(15X^2+AX+5)

=X^2(15X^2+AX^2-3X^2+5X-A/5X-1

=>A=20

X^2(x-1/5)(15X^2+20X+5)

=5X^2(X-1/5)(X+1)(3X+1)

=0