- 今天要!!!!!!!!!!!!!!!!!!
- !!!!!!!!!!!!!
- (1) (n,Sn)在C:y=-x^-3x上--->Sn=-n^-3n
--->a1=S1=-4
an=Sn-S(n-1)=-n^-3n+(n-1)^+3(n-1)=(-2n+1)-3=-2n-2
--->an=-2n-2
(2) C:y'=-2x-3--->x=-1时y'=-1,y=2--->L:y-2=-1(x+1)即:y=-x+1
--->log2_bn=-n+1--->bn=2^(-n+1)=1/2^(n-1)
--->{bn}前n项和Sn=1+1/2+1/4+...+1/2^(n-1)=2-1/2^(n-1)