a -5a+6a=0
a -2a=3a-6a
(a-2a)/(a-2a)=3
设数列b=a-2a是公比为3等比数列,b1=a2-2a1=5-2*1=3
b=3^n
a -5a+6a=0
a -3a=2a-6a
(a-3a)/(a-3a)=2
设数列c=a-3a是公比为2等比数列,c1=a2-3a1=5-3*1=2
c=2^n
a=(a-2a)-(a-3a)
=b-c=3^n-2^n