5) \(u_4=u_1+3d=5+3\cdot\left(8-5\right)=14\)
6) \(u_6=u_1+5d\Leftrightarrow27=-3+5d\Rightarrow d=6\)
7) \(u_8=u_1+7d=2+7\cdot3=23\)
\(S_8=\dfrac{8\cdot\left(2+23\right)}{2}=100\)
8) \(u_{12}=u_1+11d=-24\)
\(S_{12}=\dfrac{12\cdot\left(-24-2\right)}{2}=-156\)