2》裂项 乘除变减法1/a(n)-1/a(n+1) = 1/(6n-5)-1/(6n+1)=6/[(6n-5)*(6n+1)]=6/a(n)*a(n+1)bn = 3/(a(n)*a(n+1))=1/2*[1/a(n)-1/a(n+1)];Tn = 1/2[1/a(1)-1/a(2)+1/a(2).....-1/a(n+1)]=1/2*[1-1/(6n+1)]m>=30
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