Question

ПОМОГИТЕ ПОЖАЛУЙСТА 35 БАЛЛОВ!!!!

Answer 1

$\displaystyle\bf\\\left \{ {{a_{12} =16} \atop {a_{5} =2}} \right. \\\\\\-\left \{ {{a_{1}+11d=16 } \atop {a_{1} +4d=2}} \right.\\ ----------\\7d=14\\\\d=2\\\\a_{1} =2-4d=2-4\cdot 2=2-8=-6\\\\\\S_{20} =\frac{2a_{1} +19d}{2} \cdot 20=(2a_{1}+19d)\cdot 10=\\\\\\=\Big[2\cdot (-6)+19\cdot 2\Big]\cdot 10= (-12+38)\cdot 10=26\cdot 10=260$