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Arithmetic sequence math help...

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asdfdsa
Posts: 1268 - Karma: 13
k so I have a word problem:

Kathryn bought a doll that decreases in value by $0.60 per year. If she bought it for $15, what will the value be 7 years after she buys it?

I'm THINKING that the formula I use is: Sn = n/2 [ 2a + (n-1)d ]

Is that correct?
a being the original value ( 15 )
d being the difference between ( 0.60 )
n being the number of years ( 7 )

Help? I got something ridiculous like 92.6.
Jan 4 2010 11:11PM
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1337
Posts: 31063 - Karma: 29,376
are you serious?
Jan 4 2010 11:13PM
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OhMyBosh
Posts: 5056 - Karma: 171
umm... 15 - (0.6 * 7)?

= $10.8


unless im missing something here?
Jan 4 2010 11:14PM
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1337
Posts: 31063 - Karma: 29,376
.6 * 7 = 4.2

15 - 4.2 =10.8
unless you want the formula, thats it
Jan 4 2010 11:15PM
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PhilboydStudge
Posts: 3431 - Karma: 206
Seriously?

Amount=15-.6n and n=7
Jan 4 2010 11:16PM
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turtles
Posts: 410 - Karma: 30
$10.80?
Jan 4 2010 11:16PM
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asdfdsa
Posts: 1268 - Karma: 13
I know the question is simple as shit, but I need to use one of the formulas given. Does the formula I gave you work?
Jan 4 2010 11:16PM
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Edew19
Posts: 365 - Karma: 13
where the hell did you get that formula from?
Jan 4 2010 11:17PM
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asdfdsa
Posts: 1268 - Karma: 13
textbook? lol


Jan 4 2010 11:19PM
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asdfdsa
Posts: 1268 - Karma: 13
was really overthinking it and chose the wrong formula, youre definitely right, thanks.
Jan 4 2010 11:21PM
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1337
Posts: 31063 - Karma: 29,376
this is the formula

but if it said it decreases by 60% every year than it would be different
Jan 4 2010 11:22PM
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asdfdsa
Posts: 1268 - Karma: 13
got another, pretty sure im overthinking it too..

Lindsay earns 30000 in her first year of employment, and recieves a 3% raise every year. Determine her salary in her 8th year.

Then, find her cummulative salary from the start of her 6th year to the end of her 12th year.
Jan 4 2010 11:25PM
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p-fo
Posts: 9372 - Karma: 6,156
multiplying the original value by 2 isnt helping. i know that for sure. try f(x)= a -dn
Jan 4 2010 11:27PM
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asdfdsa
Posts: 1268 - Karma: 13
any idea what to do for the second one?
Jan 4 2010 11:31PM
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Mac.
Posts: 953 - Karma: 24
The formula for the second is 30,000*(1+.03x) where x is the amount of years, now just plug in the values.

Do the same for the second, just plug in the values, make sure to include the 6th and 12th year.
Jan 4 2010 11:50PM
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Chickendodo
Posts: 1426 - Karma: 33
Unless it's increasing 3% of the original value each year, then this won't work.  I think the formula you're looking for is

Pe^(rt)

Where P = 30,000
r (Rate) = .03
and t (Time) = 8 years

So, 30,000e^(.03*8) = 38,137.4745

The second part is a little trickier, it's late so I can't really think of an easy way to do this, but I'd just use the equation I gave, and use it for every year from 6-12 and add them all together... I know there has to be an easier way, but I can't think of it right now.
Jan 5 2010 12:04AM
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asdfdsa
Posts: 1268 - Karma: 13
thanks to both
Jan 5 2010 12:16AM
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