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Give exact ways of avoiding loss-of-significance errors in the following computations:

a. $log(x+1)-log$, with large $x$

b. $\frac{1-cosx}{x^2}$, with $x\approx 0$

c.$(1+x)^{1/3}-1$, with $x\approx 0$

Am I doing this correctly?

a. I got $$log(x+1)-log(x)= log(\frac{x+1}{x})= log(1+\frac{1}{x}) \approx \frac{1}{x}-\frac{1}{2x^2}+\frac{1}{3x^3} ...$$

c. I got $$\frac{x}{(1+x)^{2/3}+(1+x)^{1/3}+1}$$

b. I am stuck

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The question does not ask for the limits. – André Nicolas Feb 4 at 9:04
You aren't being asked to find limits, but to carry out computations. Sure, $\log1000000-\log999999$ is near zero, but how do you get a good handle on it? – Gerry Myerson Feb 4 at 9:05
3  
If for example you have a calculator that computes correctly to $8$ significant figures, but no more, and you calculate $\sqrt[3]{1+x}$, where $x=10^{-7}$, the answer is very close to $1$, so when you subtract $1$ you don't get $8$ figure accuracy. How can you rearrange the calculation to get close to $8$ figure accuracy? – André Nicolas Feb 4 at 9:14
@AndréNicolas I knew what I was doing was wrong. I should have not listened to my roommate. Is my revised answer correct? – Lays Feb 4 at 9:23
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Those look like good ways to rearrange the terms for more accurate computations. In c) you've left out some parentheses. In b) you could try to make use of $\sin^2x+\cos^2x=1$. – Gerry Myerson Feb 4 at 9:56
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2 Answers

up vote 1 down vote accepted

a)

$$\log{(x+1)} = \log{x} + \log{\left (1 + \frac{1}{x} \right )}$$

Use the Taylor series approximation for $\log{y}$ for small values of $y$:

$$\log{(x+1)} - \log{x} = \frac{1}{x} - \frac{1}{2 x^2} + \frac{1}{3 x^3} + \dots$$

b) Again, use the Taylor series for $\cos{x}$ about $x=0$:

$$\frac{1-\cos{x}}{x^2} = \frac{1}{2} - \frac{x^2}{24} + \dots$$

c) Taylor expansion or a binomial expansion:

$$\begin{align}(1+x)^{1/3}&= 1 + \frac{1}{1!} \frac{1}{3} x + \frac{1}{2!} \frac{1}{3} \left (-\frac{2}{3} \right ) x^2 + \frac{1}{3!} \frac{1}{3} \left (-\frac{2}{3} \right ) \left (-\frac{5}{3} \right ) x^3 + \ldots \\ \end{align} $$

so that

$$(1+x)^{1/3} - 1 = \frac{1}{3} x - \frac{1}{9} x^2 + \frac{5}{81} x^3 - \ldots$$

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up vote 1 down vote

As for part (c), You can use the Taylor series to get: $$(1+x)^{1/3}\sim 1+\frac{x}{3}-\frac{x^2}{9}+O(x^3)$$

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