why dosen't out put as short answer?

Question

sadiqah aldahan 2022-4-15

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1697800-why-dosen-t-out-put-as-short-answer

评论： Walter Roberson 2022-4-16

采纳的回答： Star Strider

在 MATLAB Online 中打开

syms y

R1=(nthroot(y-2,3)+2);

R2=y;

R3=1;

eq1=int(R1^2,y,1,2.1);

eq2=int(R2^2,y,2.094,4);

eq3=int(R3^2,y,0,1);

format short

volume=(eq1+eq2+eq3)*pi

volume =

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Answer 1

Star Strider 2022-4-16

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1697800-why-dosen-t-out-put-as-short-answer#answer_943925

在 MATLAB Online 中打开

Because it is symbolic, not numeric.

To get a numeric result use either vpa or double —

syms y

R1=(nthroot(y-2,3)+2);

R2=y;

R3=1;

eq1=int(R1^2,y,1,2.1);

eq2=int(R2^2,y,2.094,4);

eq3=int(R3^2,y,0,1);

format short

volume=(eq1+eq2+eq3)*pi

volume =

vpa_volume = vpa(volume)

vpa_volume =

67.308283557712918056555924159997

double_volume = double(volume)

double_volume = 67.3083

If you want to always have this sort of result, see the documentation on sympref, and specifically Display Symbolic Results in Floating-Point Format.

.

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Walter Roberson 2022-4-16

The purpose of solve() and int() are to return indefinitely precise answers whenever possible. solve() avoids approximate numeric solutions, and int() never uses approximate numeric solutions. If you int() and the exact theoretical solution is

then int() will return that, not 1.236<something>

If what you want is a numeric approximation then you should be reconsidering whether you should be using int() at all: perhaps you should be using the pure-numeric integral() or perhaps you should be doing extended precision numeric approximation using vpaintegral()

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