How to simplify symbolic calculation

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CEM CAKMAK
CEM CAKMAK 2018-10-29
评论： Walter Roberson 2018-10-29
Hi there everybody,
I have been trying to do symbolic calculation on Matlab, but the solution gives very unnecessary number which is described in below: "6.123e-17*qdot1 + 1.0*qdot2 + 1.0*qdot3 + qdot5 - 6.123e-17*cos(q5)*(3.749e-33*qdot1..."
This is an answer matrix column. I'd like Matlab to calculate 6.123e-17*qdot as 0 and like to avoid 1.0*qdot3. Could you help me about this problem?
Thanks in advance.
Have a nice day.
Best regards, Cem ÇAKMAK

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回答（1 个）

madhan ravi
madhan ravi 2018-10-29
  4 个评论
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Walter Roberson
Walter Roberson 2018-10-29
Those were not the ones I was thinking of; the one I was thinking of was quite similar to the current question.
Unfortunately I cannot locate it at this time.
The general idea is that it is possible to use evalin(symengine) or feval(symengine) to run some MuPAD code which will examine the structure of the expressions, locate constants with sufficiently small absolute value, and substitute zeros for them. This would probably involve using the MuPAD match() and subsex() and a loop.
John's answer that you linked to is still important: for sufficiently large numeric replacement values for symbolic variables, neglecting "small" contributions becomes risky.

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