Question

СРОЧНО НАДО!!! ДАЮ 40 баллов!!!!

Answer 1

$a_{11}=27\\\\a_{11}=a_{1}+10d\\\\10d=a_{11}-a_{1}=27-7=20\\\\d=2\\\\a_{20}=a_{1}+19d=7+19*2=7+38=45\\\\S_{20}=\frac{a_{1}+a_{20}}{2}*20=(7+45)*10=520\\\\Otvet:\boxed{S_{20}=520}$

$2)\left \{ {{a_{15}=16 } \atop {a_{5}=58 }} \right.\\\\-\left \{ {{a_{1}+14d=16 } \atop {a_{1}+4d=58 }} \right.\\ -------\\10d=-42\\d=-4,2\\\\a_{1}=58-4d=58-4*(-4,2)=58+16,8=74,8\\\\S_{20}=\frac{2a_{1}+19d }{2}*20=(2*74,8+19*(-4,2))*10=(149,6-79,8)*10=65,2*10=652\\\\Otvet:\boxed{S_{20}=652}$

$3)a_{15}=52\\ d=4\\\\a_{15}=a_{1}+14d\\\\a_{1}=a_{15}-14d=52-14*4= 52-56=-4\\\\S_{15} =\frac{a_{1}+a_{15}}{2}*15=\frac{-4+52}{2}*15=\frac{48}{2}*15=24*15=360\\\\Otvet:\boxed{S_{15}=360}$