请问,an=1/3*(1/3)^n-1=1/3^n怎么化出来的?Sn=(1/3*(1-1/3^n)/(1-1/3)=(1-1/3^n)/2怎么化出来的?

数列｛an},a1=1,an+1=2an-n^2+3n,求｛an｝. 关于等差数列等比数列的计算问题.1.已知数列an满足an+1=an+3n+2,且a1=2,求an0 因为an+1=an+3n+2,所以an-an-1=3n-1(n>=2)所以an=(an-an-1)+(an-1-an-2)+……+（a2-a1）+a1=3n-1+3n-4+……+5+2请问,an-an-1不是应该等于d吗? 请问数列an=n/(2n+1)如何求和 数列{an}中,a1=2,an+1=4an-3n+1,求证{an-n}是等比数列 4an中n为下标an+1中n+1为下标an-n中an的n为下标 递推法求通项如an+1 -an=3^n,求an通项哦打错了.是an+1 -3an=3^n 已知数列{an}中,a1=1,an+1=3an/(an+3)(N∈N*),求通项an, 设an=(1/n+1)+(1/n+2)+(1/n+3)+...+1/2n,则an+1-an等于? 数列{an}中,a1=3,an-an*an+1=1(n∈N*),An表示数列{an}的前n项之积,则A2009 数列an满足a1=1/3,Sn=n(2n-1)an,求an 数列{an},a1=3,an*a(n+1)=(1/2)^n,求an a1=1,an+1=an+3^n-n,求an与sn 在数列｛an｝中,a1=3,a(n+1)=an+n,求an 已知数列｛an｝中,a(n+1)=an+2^n,a1=3,求an an+1 ,n+1是下表 ,an+1=an^3 a1=2 ,求an a1=1,an+1=an/(3+an),求an=?（n为下标） 数列｛an｝满足a1=1,且an=an-1+3n-2,求an 在数列an中,a1=2 an+1=an+3n则an= 已知{an}满足a1=1.an=3*n+2An-1,求an