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I know there is a built-in function for that but can't remember what it is.

Let's say I have a number, 10, and over a period of a year that number has increased by 100% to 20. I would like to figure out what the monthly increase should be for 10 to reach 20 over 12 months.

If I convert 100% to a coefficient I get 2. Now by trying several numbers manually I figured the monthly coefficient should be around 1.065:

M.  NUMBER          COEFF
1   10              1.065
2   10.65           1.065
3   11.34225        1.065
4   12.07949625     1.065
5   12.8646635063   1.065
6   13.7008666342   1.065
7   14.5914229654   1.065
8   15.5398654581   1.065
9   16.5499567129   1.065
10  17.6257038992   1.065
11  18.7713746527   1.065
12  19.9915140051   1.065

Q: How can I obtain the monthly coefficient (something around 1.065) from 2 and 12?

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closed as off topic by Bruno Pereira, Jorge Castro, htorque, jokerdino, nitstorm Aug 17 '12 at 9:17

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2  
You may want to ask this over at math.stackexchange.com instead. –  WarriorIng64 Feb 1 '12 at 9:00

1 Answer 1

up vote 3 down vote accepted

its more a mathematical question:

so over m months you want to multiply your value x by factor f. This can be written as follows (c = coefficient per month):

f * x = c*c*...*c*x
f * x = c^m * x
f     = c^m
c     = m-based root of f

so in your example you can calculate it as:

c     = 11-based root of 2

But - this is not a mathematical Q & A, i guess this topic will be closed soon ;)

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Thanks. do you happen to know the name of the function that corresponds to m-based root of ? –  user359650 Feb 1 '12 at 9:01
    
"m-based root of x" = x ^ (1/m) - if i remember correctly it would be POW(x, (1/m)) –  Michael K Feb 1 '12 at 9:03
    
Actually calc allows me to enter x ^ (1/m) directly. However if I do 2 ^ (1/12) I get 1.0595... which doesn't seem correct as after applying that coefficient I get 18.88 instead of 20 after a period of 12 months. –  user359650 Feb 1 '12 at 9:09
    
you are right - there are only 11 increases - so it should be corrected to factor 11 ;) so use 11-th root –  Michael K Feb 1 '12 at 9:12
    
You're the man :) –  user359650 Feb 1 '12 at 9:19

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