[tex]\displaystyle\bf\\a_{1} =-40\\\\a_{20} =55\\\\\\S_{20}=\frac{a_{1} + a_{20} }{2} \cdot 20=(a_{1} + a_{20} )\cdot 10=(-40+55)\cdot 10=\\\\\\=15\cdot 10=150\\\\\\Otvet \ : \ S_{20} =150[/tex]