# Math Help - 2 question i want check the answer .

1. ## 2 question i want check the answer .

Hi
2 question i want check the answer .

thanks

2. 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 ....?

3. 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.

4. 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)

5. Originally Posted by Wilmer
In other words, n = 20 (part 1) and n = 5 (part 2)
That is what I got.

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