Problem using diff comand

2015 1 25
1 个回答
更新时间：2015 1 25
7 次查看（30 天）

0 个投票

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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回答（1 个）

Roger Stafford
Roger Stafford 2015-1-25

0 个投票

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?

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