其中n=1时,a1/b1=S1/T1=5/-1=-5,
a1=-5b1
设公差分别为d1,d2
Sn=n(a1+an)/2=na1+n(n-1)d1/2
Tn=n(b1+bn)/2=nb1+n(n-1)d2/2
Sn/Tn=[2a1+(n-1)d1]/[2b1+(n-1)d2]=(2n+3)/(3n-4)
分别令n=1,2,3代入得:
a1/b1=-5
(2a1+d1)/(2b1+d2)=7/2
(2a1+2d1)/(2b1+2d2)=9/5
a1=-5b1
d1=-4b1
d2=-6b1
a20=a1+19d1=-81b1
b20=b1+19d2=-113b1
a20/b20=81/113