Series questions Help needed (1 Viewer)

john1464 · Mar 3, 2015

1) 8-4+2-.... Which term of the following series is 1/128

2)Find the term of the series 2/7+6/7+18/7+... that is greater than 100

Drsoccerball · Mar 6, 2015

john1464 said:
1) 8-4+2-.... Which term of the following series is 1/128

2)Find the term of the series 2/7+6/7+18/7+... that is greater than 100

For question one
$a_1=8, $ Let the rate be $ \frac{1}{2}$

$\frac{1}{128} =8(\frac{1}{2})^{n-1}$

$\frac{log(\frac{1}{128\times8})}{log(\frac{1}{2})}=n-1$

$n-1 =10$

$\therefore n=11$

For question 2:

$a_1= \frac{2}{7}, r=3$

Same as question 1

$100=\frac{2}{7} (3)^{n-1}$

$$ By solving you get n=6.3333 $ \therefore n=7$

Drsoccerball · Mar 7, 2015

Do you get it?..

Mikes · Mar 8, 2015

Ployixi said:
Actually don't worry, I solved it.

Isn't the rate -0.5? Could you please explain how you solved it?

Drsoccerball · Mar 9, 2015

Mikes said:
Isn't the rate -0.5? Could you please explain how you solved it?

yes the rate is -1/2 however, you cant do log of a negative number. Besides the number that they wanted was positive so letting the rate = 1/2 doesnt affect the solution

Series questions Help needed (1 Viewer)

john1464

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Drsoccerball

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Drsoccerball

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Mikes

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Drsoccerball

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