Question

Найти n, если а33 = 17, аn = - 28, d = 3.

Answer 1

$a_{n}=a_{1}+d\cdot (n-1)\\\\a_{33}=a_{1}+d\cdot(33-1)\\\\d=3;\\\\a_{33}=17\\\\17=a_{1}+3\cdot32\\\\a_{1}=-79\\\\a_{n}=-28; a_{1}=-79; d=3\\\\a_{n}=a_{1}+d\cdot (n-1)\\\\-28=-79+3\cdot (n-1)\\\\3\cdot (n-1)=-28+79\\\\3\cdot (n-1)=51\\\\n-1=17\\\\n=18$