Wrong result from calculation

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Veselin Manev
Veselin Manev 2020-6-9
评论： Veselin Manev 2020-6-9
Dear all,
I have a question about high precision calculation of the fraction 1/998001 in Matlab. I use
vpa(1/998001,200)
which produces the following result
0.000001002003004005005955287066314596255978131011943332850933074951171875
On the Numberphile channel on youtube, they prove that the result should be
0.000001002003004005006007 .. 996997999
which has higher precision than the obtain result in MATLAB and it appears that there is some numerical error. Do you have any idea why is that?
Thank you
Regards,
Veselin

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回答（1 个）

Steven Lord
Steven Lord 2020-6-9
By the time you call vpa, MATLAB has already performed the division in double precision. So you're displaying the double precision value that results from that division to 200 decimal places.
Perform the division symbolically.
x1 = vpa(1/998001, 200)
x2 = vpa(sym(1)/998001, 200)

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