Question

calcule a razão de uma P.A sabendo que A1=100 e A21=-40

Answer 1

$a_1=100\\a_{21}=-40\\ n=21\\ r=?\\\\\\ a_n=a_1+(n-1)\times r\\\\\\ -40=100+(21-1)\times r\to\\\\ -40=100+20 r\to\\\\ -40-100=20r\to\\\\ -140=20r\to\\\\ - \dfrac{140}{20} =r\to\\\\ \boxed{r=-7}$