数学必修五的1已知等差数列 ｛an｝满足：a3=7,a3+a7=26(1)求通项公式an及前N项和sn；（2）令bn=1

1 a7=26-a3=26-7=19
d=（a7-a3)/(7-3)=3
a1=a3-2d=7-2*3=1
an=2n-2 Sn=n*1+{n*(n-1)/2}*3
2 bn=1/{（an+1)(an-1)}=1/{(2n-1)(2n-3)}
用裂项求和算Tn
Tn=-0.5(1+1)-0.5(1/3-1)-0.5(1/5-1/3)-.-0.5{1/(2n-1)-1/(2n-3)}
=-0.5{1+1/(2n-1)}