geometric seq + logarithms

[1141]

I'm wondering if anyone could help me:

Find the number of terms in each of these geometric progressions:

b.) 1,-3,9, ... ,531 441
d.) 5,-10,20, ... ,-40 960

This is my working out for b.) :

u[sub:1xyd3vd2]i[/sub:1xyd3vd2] = ar[sup:1xyd3vd2]i-1[/sup:1xyd3vd2]
531 441 = 1 x (-3)[sup:1xyd3vd2]i-1[/sup:1xyd3vd2]
531 441 = -3[sup:1xyd3vd2]i[/sup:1xyd3vd2]

log(531 441) = ilog(-3)
i = log(531 441) / log(-3)

Then I used logarithms to get i, but then I couldn't go any further because I can't log negative numbers. My calculator just says Error.

For question d.) I did everything until the log stage because once again I can't log negative numbers. What do I do?

 

[galactus]

Then don't use negatives. Pretend they're all positive and go from there.

From the first one, it is obvious they are multiplying by -3. Just use 3 instead.

There are 13 terms: 1,3,9,27,81,243,729, 2187,6561, 19683,59049, 177147, 531441

If we multiply by -3 every other tem is negative. That's the only difference.