Lời giải:
a. Với công sai d thì $u_n=u_1+d(n-1)$
Khi đó:
$2u_2+u_3-u_5=6$
$2(u_1+d)+(u_1+2d)-(u_1+4d)=6$
$2u_1=6$
$u_1=3$
$2u_4-u_6=1$
$2(u_1+3d)-(u_1+5d)=1$
$u_1+d=1$
$d=1-u_1=1-3=-2$
b.
$S_{12}=u_1+u_2+...+u_{12}$
$=u_1+(u_1+d)+(u_1+2d)+....+(u_1+11d)$
$=12u_1+(1+2+...+11)d$
$=12.3+\frac{11.12}{2}.(-2)=-96$