Question

12 и 13 задание во вложении

Answer 1

$12)a_{n}=5n-2 \\\\a_{1}=5*1-2=3\\\\a_{20}=5*20-2=98\\\\S_{20}=\frac{a_{1}+a_{20}}{2}*20=(a_{1}+a_{20})*10=(3+98)*10=1010\\\\Otvet:\boxed{1010}$

$13)a_{1}=-3\\\\d=4\\\\S_{n}=150\\\\n=?\\\\S_{n} =\frac{2a_{1}+d(n-1)}{2}*n\\\\150=\frac{2*(-3)+4(n-1)}{2}*n\\\\(- 3+2(n-1))*n=150\\\\(-3+2n-2)*n=150\\\\(2n-5)*n=150\\\\2n^{2} -5n-150=0\\\\D=(-5)^{2}-4*2*(-150)=25+1200=1225=35^{2}\\\\n_{1}=\frac{5+35}{4}=10\\\\n_{2}=\frac{5-35}{4} =-7,5-neyd$

Ответ : Надо взять 10 первых членов