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April 19, 2014

April 19, 2014

Posted by **s** on Monday, November 17, 2008 at 10:27pm.

first i did

S=18/ (1+1/3)= 13 1/2

Sn= 18(1+(1/3)^6)

_____________

1 + 1/3

= 13 14/27

13 1/2 - 13 14/27 = -1/54

what did i do wrong?

- math -
**Reiny**, Monday, November 17, 2008 at 11:43pmyour formula for Sn is wrong

it is

Sn = a(1 - r^n)/(1-r)

= 18(1 - (-1/3)^6)/(1+1/3)

= 18 (1 - 1/729)/(4/3)

= 18(728/729)(3/4)

= 364/27

((you had 365/27))

so the difference is │364/27 - 27/2│

= 1/54

It didn't say which way the subtraction was supposed to go, so I took the absolute value.

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