(1)设首项为a1,公差为d
a2=a1+d=3
S4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d
=16
解方程组,a1=1,d=2
an=a1+(n-1)d=1+2n-2=2n-1
(2)T2011=1/(1×3)+1/(3×5)+1/(5×7)+...+1/(4019×4021)
=1/2×2/（1×3）+2/（3×5）+2/（5×7）+...+2/(4019×4021）
=1/2×〔1-1/3+1/3-1/5+1/5-1/7+...+1/4019-1/4021
=1/2×(1-1/4021)
=1/2×4020/4021
=2010/4021