[tex]S_n=135\\a_1=31\\a_n=-4\\\\S_n=\frac{a_1*a_n}2*n\\135=\frac{31-4}2*n\\135=\frac{27}2*n /*2\\270=27n /:27\\10=n\\\\a_{10}=-4\\a_{10}=a_1+9r\\-4=31+9r\\-35=9r /:9\\-\frac{35}9=r[/tex]
[tex]Odp. \text{Ten ciag ma 10 wyrazow, roznica ciagu wynosi } r=-\frac{35}9=-3\frac89[/tex]
