⅔k-k=⅓k-⅔k=0-⅓k=...=-⅓k数列{tn}是以k为首项，-⅓k为公差的等差数列(a)t6=k+5·(-⅓k)=-⅔k(b)tn=k+(n-1)·(-⅓k)=(4-n)k/3(c)t20=-16(4-20)k/3=-16k=3
