∴.

∴.

(2) The symmetry axis of ∵ cn is perpendicular to the X axis, and the vertex is Pn.

∴ Let the equation of Cn hold.

Substitute Dn(0, n2+ 1) into the above formula to get a= 1.

The equation of ∴Cn is y = x2+(2n+3) x+N2+ 1.

∫kn = y ' | x = 0 = 2n+3，

∴,

∴==.

(3)t={y|y=﹣( 12n+5),n∈n*}={y|y=﹣2(6n+ 1)﹣3,n∈n*}，

∴S∩T=T, and the maximum number in t is a 1 = ∩ 17.

Let {an} tolerance be d, then ﹣ a10 = ﹣17+9d ﹣ (﹣ 265125. )

This led to.

∫ An ∈∴d = 12m(m∈n *)

∴d=﹣24，

∴an=7﹣24n(n∈N*,n≥2).