Question

Найдите an и d арифметической прогрессии у которой a1=-3, n=11, Sn=1452

Answer 1

$S_{11}=frac{a_{1}+a_{11}}{2}*11\\1452=frac{-3+a_{11} }{2}*11\\(a_{11}-3)*11=2904\\a_{11}-3=264\\a_{11}=267\\a_{11} =a_{1}+10d\\10d=a_{11}-a_{1}=267+3=270\\d=27$

Answer 2

почему 12d?

Answer 3

$S _{11} = frac{a_{1} +a_{11}}{2} times 11 \ 1452 = frac{ - 3 +a_{11} }{2} times 11 \ 2904 = - 33 + 11a_{11} \ 11a_{11} = 2904 + 33 \ 11a_{11} = 2937 \ a_{11} = 267$

$a _{11} = a _{1} + 10d \ 267 = - 3 + 10d \ 10d = 270 \ d = 27$

Ответ:

$a _{11} = 267 \ d = 27$