[tex]Zadanie\\\\a_1=-3\\\\r=-2\\\\\\a_n=a_1+(n-1)\cdot r\\\\a_{10}=a_1+9\cdot r\\\\ a_{10}= -3+9\cdot(-2)=-3-18=-21\\\\\\S_n=\frac{a_1+a_n}{2}\cdot n\\\\ S_{10} =\frac{a_1 + a_{10}}{2}\cdot 10\\\\ S_{10}=\frac{-3+(-21)}{2}\cdot10=-24\cdot5=-120[/tex]
