把(2n+1)an=(2n-3)a(n-1)变形an/a(n-1)=(2n-3)/(2n+1)
则a2/a1=1/5,a3/a2=3/7,a4/a3=5/9.a(n-1)/a(n-2)=(2n-5)/(2n-1),an/a(n-1)=(2n-3)/(2n+1)
把各项依次相乘,得到3/(2n-1)(2n+1)=an/a1=an
所以an=3/(2n-1)(2n+1)=(3/2)[1/(2n-1)-1/(2n+1)]
sn=a1+a2+.an=(3/2)[(1-1/3+1/3-1/5+1/5-1/7+.1/(2n-1)-1/(2n+1)]
=3/(2n+1)