# Solving for i in A=P(1+i)^n

1. Oct 23, 2007

### cjp88

I'm having some trouble understanding how to solve for i. This is the formula for compounding intrest. I'm reading this example which shows how to solve for i if you double your money in 5 years. So it gives this example:

(A/P)^1/n - 1 = i

So it gives A for 2 and P for 1, n would be 5 so the answer should be:

(2/1)^1/n - 1 = i

They shown the answer as 0.149 however I cannot get this same answer for whatever reason.

When I put in 2/1 I get 2, then 2^1/5 would be 0.4, minus 1 would make it -0.6.

If this answer is indeed right, could someone please show me what I'm missing.

2. Oct 23, 2007

### Galileo

$$2^\frac{1}{5}$$ can't be 0.4, since (0.4)^5 is way smaller than 1 (since 0.4<1).

Since 0.4 = 2/5, you must've raised 2 to the 1th (which is two) and divided that answer by 5.
If you're using a calculator, be sure to put in the brackets correctly: 2^(1/5)-1.

3. Oct 23, 2007

### cjp88

I see what the problem was, I wasn't using brackets on my calculator. If I do 2^1/5 I get the wrong answer because it does 2^1 which is 2 then divides by 5 to get 0.4 instead of 2^(1/5) being 1.14869.....

Thanks for the help on seeing my error.

4. Oct 23, 2007

### HallsofIvy

Yes, if you just put in "2^1/5", your calculator interprets that as (2^1)/5 = 2/5= .4.