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I'm having trouble with these type of probles (where a negative log comes up):

(All of this is solving without sigma notation)

Find the number of terms in these geometric sequences and the sum of the numbers.

11, -22, 44,....,704

I know that a1 = 11, r = -2, and an = 704, so I did:

704 = 11(-2)^n-1 so,

64 = (-2)^n-1

log 64 = (log -2) n-1

n should = 7, but when I found the log of 64 divided by the log of -2, I got .2785219413 - 1.2...

Why is there a minus sign and what am I doing wrong? Please help. Thanks!

(All of this is solving without sigma notation)

Find the number of terms in these geometric sequences and the sum of the numbers.

11, -22, 44,....,704

I know that a1 = 11, r = -2, and an = 704, so I did:

704 = 11(-2)^n-1 so,

64 = (-2)^n-1

log 64 = (log -2) n-1

n should = 7, but when I found the log of 64 divided by the log of -2, I got .2785219413 - 1.2...

Why is there a minus sign and what am I doing wrong? Please help. Thanks!

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