大致说一下思路,步骤简略一点：假设法,假设{an}为等差数列,有其和Sn=（a1+an）n/2,可有Sn+1=（ a1+ an+1 ）（n+1）/2= a1（n+1）/2 + an+1（n+1）/2=a1 n/2+ a1/2+ an+1/2 + an/2 +nd/2 =a1n/2 + an n/2+（a1 + an+1...
