Given that the sequence {an} is an arithmetic sequence, the sum of the first n terms Sn = n ^ 2, find the value of a1 + a3 + A5 + A7 +. + a25

an=Sn-S(n-1)=n^2-(n-1)^2=2n-1
a1=1,a25=49
A1, a3-a25 are odd terms, 13 terms in total
a1+a3+a5+a7+.+a25=(a1+a25)*13/2=25*13

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