I have gotten the rest of the problems for this assignment correct, but I cannot seem to figure these two out. They are in WebAssign 5 from MA 132. Any help would be greatly appreciated!!


Consider a savings plan with an initial deposit of $2300, an annual interest rate 3.5% compounded monthly and a monthly deposit of $170.

Please note that in this assignment you should round the interest rate to no less than 5 or 6 decimal places. Rounding to less than 5 or 6 decimal places will give incorrect answers. When entering the interest rate in any formulas within your spreadsheet you will get more accurate answers if you (for example) enter 10.25% over a period of 12 months as 0.1025/12 instead of 0.00854.


Write the formula derived by the Algebraic Method with the numbers appropriate for this problem.

(Symbolic) Note: Use the variable n
yn =

Use the formula derived by the Algebraic Method to find the savings after four years (48 months).

[Edited on October 14, 2013 at 12:23 PM. Reason : added homework #]

10/14/2013 12:22:22 PM

http://themathblogger.com/2009/12/05/compound-interest-w-monthly-deposits/

10/14/2013 2:52:54 PM

I used that formula to plug my numbers in... didn't work out. I ended up with a 2 million something number which is far too large of a number. The answer for the solution in the example from that link is 10 million something, so I think they did something wrong in setting that up.

10/14/2013 5:28:44 PM

there is a mistake on that page. instead of

F = D( ( (1 + r/n)nt – 1) / (r/n) ) + B(1 + r/n)nt

it should say

F = D( ( (1 + r/n)^nt – 1) / (r/n) ) + B(1 + r/n)^nt

the nt should be a power, which in your case would be 48 (months).

here is the formula written in a better way:
http://www.patrickschneider.com/blog/2008/03/compound-interest-with-an-initial-balance-and-monthly-contributions/

and a compound interest calculator you can use to check your answers:
http://www.patrickschneider.com/compound_interest_calculator.php

and remember, google is your friend. i searched for compound interest with monthly contributions and that's the first result i got.

10/14/2013 7:14:52 PM

thank you so much

10/14/2013 10:00:00 PM

fucking webassign man. i was like why the fuck am i in raleigh for this shit? i could be doing this from my beach house. its not like im @ duke or better where the prof might actually say something novel.

10/15/2013 7:47:35 AM

I wish you would say something novel.

10/15/2013 12:05:26 PM

Should I even mention, at this point, the fact that somebody's cheating on webassign with their unity id?

10/18/2013 1:59:10 AM

i don't know, we just pointed him to links on the web. you are allowed to google, aren't you, for webassigns?

10/18/2013 10:31:02 PM

His Google-Fu is weak.

10/22/2013 10:31:20 AM