Question

calcule a soma dos 20 primeiros termos da P.A(-7, -3, 1...)​

Answer 1

Resposta: 620

$r=a_{2}-a_{1}\\r=-3-(-7)\\r=7-3\\r=4\\\\a_{20}=a_{1}+19r\\a_{20}=-7+(19*4)\\a_{20}=76-7\\a_{20}=69\\\\\\soma;\\S_{n}=\frac{(a_{1}+a_{n})*n}{2}\\\\S_{n}=\frac{(-7+69)*20}{2}\\\\S_{n}=\frac{62*20}{2}\\\\S_{n}=\frac{1240}{2}\\\\S_{n}={620}$

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