a5+a4-a3-a2=a5-a3+a4-a2=a3(a2-1)+a2(a2-1)=(a2-1)(a3+a2)=(a+1)(a-1)a2(a+1)
a3-1=(a-1)(a2+a+1)
相除约掉(a-1)
=a2(a+1)2/(a2+a+1)
={a(a+1)}2/(a2+a+1)
=[{a2+a}2-1+1]/(a2+a+1)
={(a2+a+1)(a2+a-1)+1}/(a2+a+1)
=a2+a-1+{1/（a2+a+1）}