# Trouble graphing compound interest

Marcie Monaco shared this question 3 years ago

I am trying to graph a compound interest problem with sliders. I've tried it multiple times and I can't figure out what I'm doing wrong. The formula looks correct but it keeps appearing in the wrong quadrant. I attached a geogebra file and copied the parameters for the assignment below if anyone cares to walk me through where my error lies.

MM

The assignment:

Your task is to create a GeoGebra file that will calculate and graph financial investment amounts based on the two types of exponential growth functions studied in this chapter (i.e. investments that are compounded a specific number of times per year (e.g. quarterly, monthly, daily, etc.) and investments that are compounded continuously).

Your independent variable (x) will represent time (in years).

Your dependent variable (y) will represent the total accumulated amount of money (i.e. the final amount).

You will need to create THREE sliders to represent the three following three variables:

p = principal (starting amount of the investment). Allow your "p" slider to have a range from \$0 to \$200,000 with an increment of \$1.

r = interest rate. Allow your "r" slider to have a range from 0% to 15% with an increment in hundredths of a percent. (Remember, r needs to be represented as a decimal in your calculations, not as a percent. For example, if you want to do 8.75% you would need r = 0.0875).

n = number of times compounded in a year

Your GeoGebra should have TWO separate functions. One for investments compounded a specific number of times per year (i.e. one that uses the "n" slider).....and ONE for investments that are compounded continuously (i.e. one that will not use the "n" slider).

The final GeoGebra file should allow the user to simply adjust the sliders for p, r, and n to any desired quantities......and then display the functions and the graphs of the exponential growth of the funds. 1

The formula appears in 2nd and 3rd quadrant because it's the natural domain of an exponential function.

In your case, the formula refers to time, so time is supposed to be a variable >= 0.

1. it's better to change the axis scale, because you're supposed to see what happens to your money in a few years, not in the next 10000 years! :) Select the Move Graphics View tool, then drag the x-axis (which represents time) until you get a nice scaling. (I've done it for you)

2. you should show only the portion of your function that refers to positive values of the variable x. It's the red function in my file. When you want to restrict the domain of an already existing function, just type in input bar the name of the function, followed by a comma, and the interval, e.g. if your function is f(x) and you want it displayed for 2<x<3, then type in input bar

`f(x), 2<x<3`