Question

Calcular o oitavo,o nono e o décimo primeiro termo da p.a cujo primeiro termo é 4 e a razão é -2

Answer 1

Utilizando a relação do termo geral da PA  $\boxed{a_n~=~a_1+(n-1)\cdot r}$ , temos:

$\underline{8^o~Termo}:\\\\a_8~=~a_1+(8-1)\cdot r\\\\a_8~=~4+7\cdot(-2)\\\\a_8~=~4-14\\\\\boxed{a_8~=\,-10}\\\\\\\underline{9^o~Termo}:\\\\a_9~=~a_1+(9-1)\cdot r\\\\a_9~=~4+8\cdot(-2)\\\\a_9~=~4-16\\\\\boxed{a_9~=\,-12}\\\\\\\underline{11^o~Termo}:\\\\a_{11}~=~a_1+(11-1)\cdot r\\\\a_{11}~=~4+10\cdot(-2)\\\\a_{11}~=~4-20\\\\\boxed{a_{11}~=\,-16}$