如图!如图!
如图!s1=a1(a1+1)/2=a1 an正数得a1=1Sn-Sn-1=an所以 an (an+1)/2-an-1(an-1+1)/2=an整理下 (an-an-1)(an+an-1)=(an+an-1)得an+an-1=0 或an-an-1=1但an正数,所以an+an-1=0 不成立所以an-an-1=1 又因为a1=1所以an=n