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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec. 1] Operations on Approximate Numbers 369<br />

xitnate numbers a t , . . . , a n , then the limit<strong>in</strong>g absolute error AS of the resuU<br />

may be evaluated approximately<br />

S = /(a t, ..., a n )<br />

from the formula<br />

The limit<strong>in</strong>g relative error S is then equal to<br />

".++<br />

,**-<br />

*Ji/r<br />

Example 4. Evaluate S = ln (10.3+ V^4. 4 ); the approximate numbers<br />

10.3 and 4.4 are correct to one decimal place.<br />

Solution. Let us first compute the limit<strong>in</strong>g absolute error AS <strong>in</strong> the<br />

general form: S = ln (a+ V r 1<br />

~b), AS = A& \<br />

Aa<br />

We have<br />

^TT;<br />

Aa=A6=i=2Q; 1^4.4 = 2.0976...; we leave 2.1, s<strong>in</strong>ce the relative error of<br />

the approximate number y is equal to ^ =- -r= ^ ;<br />

is then equal to ^SOTT^JQ<br />

20 "2"<br />

the absolute error<br />

' we can be sure f tne first decimal place. Hence,<br />

Thus, two decimal places will be correct.<br />

Now let us do the calculations with one reserve decimal:<br />

0005 U - UUi> '<br />

log (10.3+ |/4 4) =5= log 12 4-1.093, In (10 3+ V 4.4)=^ 1.093-2.303 = 2.517.<br />

And we pet the answer: 2 52<br />

8. Establish<strong>in</strong>g admissible errors of approximate numbers for a given<br />

error <strong>in</strong> the result of operations on them. Apply<strong>in</strong>g the formulas of 7 for<br />

the quantities AS or 6S given us and consider<strong>in</strong>g all particular differentials<br />

U-M Ac* or the quantities \-~- equal, we calculate the admissible<br />

\dak \<br />

* M<br />

\dak \f\<br />

absolute errors Aa lt ... , Aa^, ... o the approximate numbers a t , . .. ,an , ...<br />

that enter <strong>in</strong>to the operations (the pr<strong>in</strong>c<strong>ipl</strong>e of equal effects).<br />

It should be po<strong>in</strong>ted out that sometimes when calculat<strong>in</strong>g the admissible<br />

errors of the arguments of a function it is not advantageous to use the<br />

pr<strong>in</strong>c<strong>ipl</strong>e of equal effects, s<strong>in</strong>ce the<br />

practically unfulfilable In these cases<br />

latter may make<br />

it is advisable to<br />

demands that are<br />

make a reasonable<br />

redistribution of errors (if this is possible) so that the overall total error does<br />

not exceed a specified quantity. Thus, strictly speak<strong>in</strong>g, the problem thus<br />

posed is <strong>in</strong>determ<strong>in</strong>ate.<br />

Example 5. The volume of a "cyl<strong>in</strong>drical segment", that is, *a solid cut<br />

off a circular cyl<strong>in</strong>der by a plane pass<strong>in</strong>g through the diameter of the base<br />

(equal to 2R) at an angle a to the base, is computed from the formula<br />

2<br />

V =~ R* tan a. To what degree of accuracy should we measure the radius

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