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

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- Mar 5th 2010, 09:27 AMr-soy2 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 - Apr 23rd 2010, 04:58 AMmr fantastic
- Apr 23rd 2010, 09:55 PMpflo
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:

$\displaystyle a_n = a_1 + (n-1)d$

$\displaystyle S_n = \frac{a_1 + a_n}{2}(n)$

In both problems, you know the sum $\displaystyle S_n$ as well as $\displaystyle u_1$ and $\displaystyle d$. You can plug the first formula into the second to get rid of the $\displaystyle a_n$, which would make $\displaystyle n$ the only variable. In solving the equation for $\displaystyle n$, you should see that it is a quadratic equation: $\displaystyle 0 = \frac{d}{2}n^2 + (u_1 - \frac{d}{2})n - S_n$. 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, 12:19 AMWilmer
- Apr 24th 2010, 05:04 AMpflo
- Apr 24th 2010, 06:00 AMWilmer
Yes, I realise that Pflo; wanted to make sure R-soy realised it too:

he has 79.2268 and 43, quite a ways off.