MHB Question about errors in Numerical Analysis

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In Numerical Analysis, subtracting two nearly equal numbers can lead to significant errors due to limitations in floating-point representation. For example, calculating 1.000001 - 1 results in a small value of 0.000001, but the accuracy of this result is affected by the precision of the original numbers. The inherent accuracy of each number is approximately ±0.0000005, which translates to a relative error of 50% for the result. This illustrates how small differences can lead to disproportionately large errors in numerical computations. Understanding these concepts is crucial for effective numerical analysis.
evinda
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Hey! I am looking at an exercise in Numerical Analysis and I got stuck.
Why do we have a huge error when we substact two almost equal numbers?
 
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evinda said:
Hey! I am looking at an exercise in Numerical Analysis and I got stuck.
Why do we have a huge error when we substact two almost equal numbers?

Hey evinda!

Suppose we calculate $1.000001 - 1$.
In a "float" you can only keep 7 significant digits, so the first number cannot have more reliable digits than it already has.
The accuracy of both numbers is $\pm 0.0000005$.
Or as a percentage: $$\frac{0.0000005}{1} \times 100\% = 0.00005 \%$$, which is a pretty small error.The result of the subtraction is $0.000001$, but its accuracy is still about $0.0000005$.
Or as a percentage $$\frac{0.0000005}{0.000001} \times 100\% = 50 \%$$I'd say that is quite a large error relatively speaking. Don't you?
 
Great...I got it...Thank you very much! :o
 
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