Question

7) Calcule a soma dos oito primeiros termos da P.A. (3, 15, 27......)​

Answer 1

Resposta:

360

Explicação passo-a-passo:

$\boxed{S_{n}=\dfrac{[2\times \;a_{1}+(n-1)\times \;r]\times \;n}{2}}\\\\S_{n}=\dfrac{[2\times \;a_{1}+(n-1)\times \;r]\times \;n}{2}\\\\S_{8}=\dfrac{[2\times \;3+(8-1)\times \;12]\times \;8}{2}\\\\S_{8}=\dfrac{[6+7\times \;12]\times \;8}{2}\\\\S_{8}=\dfrac{[6+84]\times \;8}{2}\\\\S_{8}=\dfrac{90\times \;8}{2}\\\\S_{8}=90\times \;4\\\\S_{8}=\boxed{360}$