数列题求通项。

(1) a1=1/4 (a1 -1), a1=-1/3

(2) an = Sn-S(n-1) = (an -1) - [a(n-1)-1]/4 = [an-a(n-1)]/4

3an = - a(n-1), an/a(n-1) = -1/3, 等比数列

an = a1*(-1/3)^(n-1) = -1/3 *(-1/3)^(n-1) = (-1/3)^n

a1=-1/3，
通项是以-1/3为首项，-1/3为等比的等比数列
an=（-1/3）^n