>> syms x;The common approximation is that as long as x is small, ln(1+x) ~= x. How close is this approximation anyways? In the following Excel table, various small and incrementing values of x were chosen, along with the actual ln(1+x) values. In all cases, the approximation yields a slightly higher answer, since the most significant term of -x^2 was left out. The difference and the % error to the actual values were calculated:
>> taylor(log(1+x))
ans =
x^5/5 - x^4/4 + x^3/3 - x^2/2 + x
x | ln(1+x) | + / - | % Error |
0.000 | 0.00000 | 0.00000 | #DIV/0! |
0.005 | 0.00499 | 0.00001 | 0.250% |
0.010 | 0.00995 | 0.00005 | 0.499% |
0.015 | 0.01489 | 0.00011 | 0.748% |
0.020 | 0.01980 | 0.00020 | 0.997% |
0.025 | 0.02469 | 0.00031 | 1.245% |
0.030 | 0.02956 | 0.00044 | 1.493% |
0.035 | 0.03440 | 0.00060 | 1.740% |
0.040 | 0.03922 | 0.00078 | 1.987% |
0.045 | 0.04402 | 0.00098 | 2.233% |
0.050 | 0.04879 | 0.00121 | 2.480% |
0.075 | 0.07232 | 0.00268 | 3.705% |
0.100 | 0.09531 | 0.00469 | 4.921% |
0.200 | 0.18232 | 0.01768 | 9.696% |
0.300 | 0.26236 | 0.03764 | 14.345% |
0.400 | 0.33647 | 0.06353 | 18.881% |
0.500 | 0.40547 | 0.09453 | 23.315% |
0.750 | 0.55962 | 0.19038 | 34.021% |
1.000 | 0.69315 | 0.30685 | 44.270% |
Observing for x is 0.05 or smaller, it seems as though the absolute error grows in a quadratic manner, while the percentage error grows constantly. But most importantly, if the approximation of ln(1+x) = x wants to stay within 5% of the actual answer, the value of x shouldn't exceed 0.1.