Giúp em câu 8 vs ạ

Câu 8:
a) Ta có:
$u_{1} =3$
$u_{2} =4\times 3-1=11$
$u_{3} =4\times 11-1=43$
$u_{4} =4\times 43-1=171$
$u_{5} =4\times 171-1=683$
a) là sai
b) Ta có:
Đặt $\displaystyle v_{n} \ =\ u_{n} \ -\frac{1}{3}$
$v_{1} =u_{1} -\frac{1}{3} =3-\frac{1}{3} =\frac{8}{3}$
$v_{2} =u_{2} -\frac{1}{3} =11-\frac{1}{3} =\frac{32}{3}$
$v_{n+1} =u_{n+1} -\frac{1}{3} =4u_{n} -1-\frac{1}{3} =4(u_{n} -\frac{1}{3} )=4v_{n}$
$\displaystyle v_{n} \ $là cấp số nhân với $\displaystyle q\ =\ 4$
b đúng

c) Ta có:
$\displaystyle v_{1} =$ $\frac{8}{3}$ ;$\displaystyle \ q\ =\ $4 nên $v_{n} =\frac{8}{3} .4^{n-1}$
$u_{n} =v_{n} +\frac{1}{3} =\frac{8}{3} .4^{n-1} +\frac{1}{3}$
c đúng
d) 
$u_{6} =4.683-1=2731$
$u_{7} =4.2731-1=10923$
$u_{8} =4.10923-1=43691$
$S_{8} =58253$
d sai