已知等比数列{an}的通项公式an=3*(1/2)^(n-1)且:bn=a(3n-2)+a(3n-1)+a(3n),求证：数列

bn=a(3n-2)+a(3n-1)+a(3n)=3*(1/2)^(3n-2-1)+3*(1/2)^(3n-1-1)+3*(1/2)^(3n-1)=3*(1/2)^(3n-3)+3*(1/2)^(3n-2)+3*(1/2)^(3n-1)=3*(1/2)^(3n-1)[(1/2)^-2+(1/2)^-1+1]=3*(1/2)^(3n-1)*(4+2+1)=21*(1/2)^(3n-1)b(n+1)=...