2a³-a²-2a+1=0
2a³-2a-（a²-1）=0
2a(a²-1)-(a²-1)=0
(2a-1)(a+1)(a-1)=0
2a-1=0,a1=1/2
a+1=0,a2=-1
a-1=0,a3=1