# 2 question i want check the answer .

• Mar 5th 2010, 10:27 AM
r-soy
2 question i want check the answer .
• Apr 23rd 2010, 05:58 AM
mr fantastic
Quote:

Originally Posted by r-soy

Can't you check your answers against those in the back of the book ....?
• Apr 23rd 2010, 10:55 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.
• Apr 24th 2010, 01:19 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)
• Apr 24th 2010, 06:04 AM
pflo
Quote:

Originally Posted by Wilmer
In other words, n = 20 (part 1) and n = 5 (part 2)

That is what I got.
• Apr 24th 2010, 07:00 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.