①∵a3a7=(a5-2d)(a5+2d)=-15∴a5²-4d²=-15∵a4+a6=2a5=-2a5=-1∴1-4d²=-15d=2∴an=a5+d(n-5)=2n-11②bn=1/n（an+13）=1/n(2n+2)=0.5×1/n(n+1)=0.5×［1/n -1/（n+1）］(裂项法)∴Tn=0.5×［1-1/2+1...