13)两边同时除以an*an+1,得1/an-1/a(n+1)=1设bn=1/an,所以b(n+1)-bn=-1,bn=b1-(n-1)=-1-n+1=-n,an=-1/n14)易知a1=1,a3=9,a5=17,...成等差数列,公差8
a2--5,a4=-13,...成等差数列,公差8又因为a1+a2=a3+a4=...=a(2n-1)+a2n=-4S15=-4*7+a15=-28+1+7*8=29S22=-4*11=-44S31=-4*15+a31=-60+1+8*15=61原式=29-44-61=-76...
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