Question

determine a soma dos elementos da pa (-8, -1, 6,...,41)

Answer 1

Bom dia,

$r = 6 - (-1)\\r = 7\\\\a_n = a_1 + (n - 1) \cdot r\\41 = - 8 + (n - 1) \cdot 7\\49 = (n - 1) \cdot 7\\n - 1 = 7\\n = 8\\\\\\S_n = \dfrac{n\cdot(a_1 + a_n)}{2}\\\\S_8 = \dfrac{8 \cdot (-8 + 41)}{2} \\\\S_8 =\dfrac{8 \cdot 33}{2} \\\\S_8 =\dfrac{264}{2}\\\\ S_8 = 132$

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