Question

PROGRESSAO ARITMETICA

numa PA, a8=16 e a10=20. calcule a1, r e a20

Answer 1

Vamos fazer um sistema

$a_8=a_1+7r\\ a_1_0=a_1+9r\\\\ 16=a_1+7r\\ 20=a_1+9r\\\\ 20-16=a_1-a_1-7r+9r\\\\ 4 = 2r\\\\ r=\frac{4}{2}\\\\ \boxed{r=2}$

$a_8=a_1+7r\\\\ 16=a_1+7(2)\\\\ 16=a_1+14\\\\ a_1=16-14\\\\ \boxed{a_1=2}$