Question

a)an=5-3n b) a3=4;a7=0 найдите S10-?

Answer 1

$a);a_n=5-3n\a_1=5-3=2\a_{10}=5-30=-24\S_{10}=frac{a_1+a_{10}}2cdot 10=(2-24)cdot5=(-22)cdot5=-110\\b);a_3=4,;a_7=0\a_3=a_1+2d\a_7=a_1+6d\begin{cases}a_1+2d=4\a_1+6d=0end{cases}Rightarrowbegin{cases}-4d=4\a_1=-6dend{cases}Rightarrowbegin{cases}d=-1\a_1=6end{cases}\S_{10}=frac{2a_1+9d}2cdot10=(12-9)cdot5=15$