[tex]\displaystyle\bf\\5)\\\\a_{1} =-28 \ \ , \ \ n=9 \ \ , \ \ S_{n}=0 \\\\\\S_{9} =\frac{2a_{1} +8d}{2} \cdot 9=\Big(a_{1}+4d\Big)\cdot 9\\\\\Big(a_{1}+4d\Big)\cdot 9=0\\\\a_{1} +4d=0\\\\4d=-a_{1}=-\Big(-28\Big)=28\\\\d=28:4=7\\\\a_{9} =a_{1} +8d=-28+8\cdot 7=-28+56=28\\\\Otvet \ : \ d=7 \ \ ; \ \ a_{9} =28[/tex]

[tex]\displaystyle\bf\\8)\\\\a_{3} + a_{18} =54\\\\a_{1} +2d+ a_{1} +17d=2a_{1} +19d\\\\\boxed{2a_{1} +19d=54}\\\\\\S_{20} =\frac{2a_{1} +19d}{2} \cdot 20=\frac{54}{2} \cdot 20=540\\\\Otvet \ : \ S_{20}=540[/tex]