d是什么?等差数列的公差吗?
A1、A2、A3、A4、A5、A6、.A(2n-1)、A(2n)
An=A1+(n-1)d
A1=An-(n-1)d
A2=A1+d
S奇=A1+A3+A5+.+A(2n-1)=n[2A1+2(n-1)d]/2=n[An-(n-1)d+(n-1)d]=nAn
S偶=A2+A4+A6+.+A(2n)=n[2A2+2(n-1)d]/2=n[A2+(n-1)d]=n[A1+nd]=n[An-(n-1)d+nd]=n(An+d)
S奇/S偶=nAn/n(An+d)=An/(An+d)
S奇-S偶=nAn-n(An+d)=-nd