已知f=2x-1,g=-2x,数列{an}(n∈正整数）的各项都为整数,其前n项和为sn,若点（a2n-1,a2n)【2n-1和2n都是下标】,在函数y=f或y=g的图线上,且当n为偶数时,an=n/2,则S80=

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