Problem using diff comand

Question

Adnan Ali 2015-1-25

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https://ww2.mathworks.cn/matlabcentral/answers/171487-problem-using-diff-comand

回答： Roger Stafford 2015-1-25

Hi, I am having problem using diff command. I am trying to differentiate following function and matlab is missing 2nd part of answer.

a=90.0 - 90.0*exp(-0.167*t)*cos(1.99*t) - 7.53*exp(-0.167*t)*sin(1.99*t); b=diff(90.0 - 90.0*exp(-0.167*t)*cos(1.99*t) - 7.53*exp(-0.167*t)*sin(1.99*t); vpa(b,3)

ans 181.0*exp(-0.167*t)*sin(1.99*t)

 it is missing 0.453e-1*exp(-.167*t)*cos(1.99*t).
Full complete answer which i found using maple is
0.453e-1*exp(-.167*t)*cos(1.99*t)+180.35751*exp(-.167*t)*sin(1.99*t).
Will you please help to figure this out. I am with Matlab.
Thank you,
Adnan

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Answer 1

Roger Stafford 2015-1-25

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在 MATLAB Online 中打开

I am guessing that your difficulty is due to the low number of digits accuracy, namely only 3, that you requested. Write

vpa(b,100)

and see what you get.

Asking for only three decimal digits will probably produce terribly inaccurate results and some terms might be missing altogether. Why have you done this?