a³-3a-18=0a³-3a²+3a²-9a+6a-18=0a²(a-3)+3a(a-3)+6(a-3)=0(a-3)(a²+3a+6)=0a²+3a+6=(a+ 3/2)² +15/4恒>0,要等式成立,只有a-3=0a=3