2 question i want check the answer .

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March 5th 2010, 09:27 AM
r-soy

2 question i want check the answer .

Hi
2 question i want check the answer .

http://www12.0zz0.com/2010/03/05/06/207886290.jpg

http://www10.0zz0.com/2010/03/05/18/277725831.jpg
thanks
April 23rd 2010, 04:58 AM
mr fantastic

Quote:

Originally Posted by r-soy

Hi
2 question i want check the answer .

http://www12.0zz0.com/2010/03/05/06/207886290.jpg

http://www10.0zz0.com/2010/03/05/18/277725831.jpg
thanks

Can't you check your answers against those in the back of the book ....?
April 23rd 2010, 09:55 PM
pflo

You are correct that these are both questions about arithmetic series. But your formulas are incorrect so your answers are wrong. You need a better undersanding of what each term of the formulas actually do. Anyway, here are the correct formulas:
$a_n = a_1 + (n-1)d$
$S_n = \frac{a_1 + a_n}{2}(n)$
In both problems, you know the sum $S_n$ as well as $u_1$ and $d$ . You can plug the first formula into the second to get rid of the $a_n$ , which would make $n$ the only variable. In solving the equation for $n$ , you should see that it is a quadratic equation: $0 = \frac{d}{2}n^2 + (u_1 - \frac{d}{2})n - S_n$ . The simpliest way to solve these will be by using the quadratic formula.

The answers I get are 20 months and 5 years.
April 24th 2010, 12:19 AM
Wilmer

Quote:

Originally Posted by pflo

The answers I get are 20 months and 5 years.

In other words, n = 20 (part 1) and n = 5 (part 2)
April 24th 2010, 05:04 AM
pflo

Quote:

Originally Posted by Wilmer

In other words, n = 20 (part 1) and n = 5 (part 2)

That is what I got.
April 24th 2010, 06:00 AM
Wilmer

Yes, I realise that Pflo; wanted to make sure R-soy realised it too:
he has 79.2268 and 43, quite a ways off.