a³－5a²＋5a－4=a³-4a²-a²+5a-4=a²(a-4)-(a²-5a+4)=a²(a-4)-(a-4)(a-1)=(a-4)(a²-a+1)
＝a^3-4a^2-a^2+5a-4 =a^2(a-4)-(a^2-5a+4) =a^2(a-4)-(a-1)(a-4)=(a-4)(a^2-a+1)
