Question

已知数列xn,其前n项和为sn,点pn的坐标为（xn,sn）,

Answer 1

(1)

kXn=Sn=X1+X2+...+Xn

kXn-1=Sn-1=X1+X2+...+Xn-1

上面两个相减得

(k-1)Xn=kXn-1

Xn=k/(k-1)Xn-1

(2)

f(k)=k/(k-1)

bn=-(bn-1)/(bn-1-1)

1/(bn)=-(bn-1-1)/(bn-1)=-1+1/(bn-1)

1/(bn)-1/(bn-1)=-1

1/(bn)-1/b1=-n+1

bn=-1/n