∴a5+a7=a3+2d+a3+4d
=2a3+6d
=14+6d
=22
即d = 4/3，a1 = a3-2d = 13/3。
an = a 1+(n-1)d = 13/3+(n-1)*(4/3)= 4n/3+3
sn =(a 1+an)n/2 =(4/3+4n/3+3)n/2 = 2n？/3+13n/6
2、bn=1/(anan+1)
= 1/[(4n/3+3)(4n/3+13/3)]
= 1/{[(4n+9)/3]*[(4n+13)/3]}
=9/[(4n+9)(4n+13)]
= 9/4 *[1/(4n+9)-1/(4n+13)]
Tn=b1+b2+……+bn
= 9/4 *(1/13-1/17)+3/4 *(1/17-1/21)+……+3/4 *[1/(4n+9)-1/(4n+13)]
= 9/4 *[1/13-1/17+1/17-1/21+……+1/(4n+9)-1/(4n+13)]
= 9/4 *[1/13-1/(4n+13)]
= 9/4 * {(4n+13-13)/[13(4n+13)]}
= 9/4 * { 4n/[13(4n+13)]}
=9n/(52n+169)