解
an=[3-a(n-1)]/2
2an=3-a(n-1)
2an-2=1-a(n-1)
2(an-1)=-[a(n-1)-1]
an-1=-1/2[a(n-1)-1]
{an-1}是一个公比是-1/2的等比数列,
an-1=(a1-1)*(-1/2)^n/[1-(-1/2)]=2/3(a1-1)*(-1/2)^n
an=2/3(a1-1)*(-1/2)^n+1
解
an=[3-a(n-1)]/2
2an=3-a(n-1)
2an-2=1-a(n-1)
2(an-1)=-[a(n-1)-1]
an-1=-1/2[a(n-1)-1]
{an-1}是一个公比是-1/2的等比数列,
an-1=(a1-1)*(-1/2)^n/[1-(-1/2)]=2/3(a1-1)*(-1/2)^n
an=2/3(a1-1)*(-1/2)^n+1