当n=1时
a1=S1=1+2-1=2
当n≥2时
a(n)=S(n)-S(n-1)
=n^2+2n-1-[(n-1)^2+2(n-1)-1]
=n^2+2n-1-(n^2-2)
=2n+1
a1+a3+a5+…+a99
=2+7+11+...+199
=2+(7+199)*49/2
=5049