# Finding a function

1. Apr 13, 2009

### jared89

write a function that starts at 20 and triples every 7

2. Apr 13, 2009

### arildno

I'm sure you have just learnt about exponential functions!

Utilize that knowledge to find a solution to your problem.

3. Apr 13, 2009

### coomast

Hello jared89, welcome to the forum

You could use the exponential function in a general form and start fitting it to the two conditions you have, however there is a slightly more advanced way. Write out the two conditions as:

$$f(0)=20$$
$$f(7x)=2f(x)$$

Take the derivative of the second one and use in it the original again. You will end up with a first order differential equation giving as solution the exponential function. Using the first equation as boundary gives then the function you are looking for.

Can you do this?