2 question i want check the answer .

**r-soy** · March 5th 2010, 09:27 AM

Hi
2 question i want check the answer .

thanks

**mr fantastic** · April 23rd 2010, 04:58 AM

Originally Posted by r-soy

Hi
2 question i want check the answer .

thanks

Can't you check your answers against those in the back of the book ....?

**pflo** · April 23rd 2010, 09:55 PM

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.

**Wilmer** · April 24th 2010, 12:19 AM

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)

**pflo** · April 24th 2010, 05:04 AM

Originally Posted by Wilmer

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

That is what I got.

**Wilmer** · April 24th 2010, 06:00 AM

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

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