[tex]\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\la\la\la\la\ddddddddddddddddddddddddddddddddcleverdddddd\ffffffffffffffffffffffffffffffffffffffff\pppppppppppppppppppppppppppppppppppp\ddddddddddddddddddd \displaystyle \Large \boldsymbol{} a_1=70 \ ; \ d=-5 \\\\S_n=\frac{a_1+a_n}{2} \cdot n \ ; \ a_n=a_1+(n-1)d \Longrightarrow \\\\\\ S_{30}=\frac{70+70-29\cdot5}{2} \cdot 30 =-75 \\\\\\Answer: S_{30}=-75[/tex]
