a（n+1）-3an+2a（n-1）=0a(n+1)-an=2[an-a(n-1)][a(n+1)-an]/[an-a(n-1)]=2{an-a(n-1)}是公比为2的等比数列an-a(n-1)=(a2-a1)*2^(n-2)=(2-1)*2^(n-2)= 2^(n-2)a(n-1)-a(n-2)=2^(n-3)...a2-a1=2^(0)两边相加得：an-a1=2^(n-2)
