bn=1/n(a1+a2+...+an)
=1/n *(a1+an)n/2
=(a1+an)/2
=a1+(n-1)d/2
设cn=an-bn=(1-n)d/2=d/2-nd/2
S25-T25=c1+c2+...+c25
=25*d/2-(1+2+..+25)d/2
=-(1+2+...+24)d/2
=-150d∈(0,1),
-1/150
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