Write the explicit formula for the geometric sequence. a1 = -5 a2 = 20 a3 = -80 A. an = -5 • (-4)n B. an = -5(-4)n-1 C. an = -4(-5)n-1 D. an = -5 • (4)n

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look a1 = -5 n=1 a2=20 n=2, etc..

It would be B because in a geometric sequence, you have \[\large ar ^{n-1}\] and in finding our r, we take \(\large\frac{ a _{2} }{ a _{1} }\) and that is -4

This way you can eliminate A because term 2 doesn't match up... And I think Jhanny's right

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