Question

Calculate the sum of the first 30 terms in the sequence that begins with 70,65,60

Answer 1

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