subs(): Unexpected simplification behaviour

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Thomas Bauer
Thomas Bauer 2015-4-27
回答： Philip Caplan 2015-4-29
syms Gr Ra Ri A B Gr1 Gr2 c d
Gr = 2*c*(Ra^2 - Ri^2) - d*(Ra^4 - Ri^4)/2;
Gr1 = subs(Gr, [c*(Ra^2 - Ri^2), -d*(Ra^4 - Ri^4)/2], [A, B])
Gr2 = subs(Gr, [c*(Ra^2 - Ri^2), d*(Ra^4 - Ri^4)/2], [A, B])
returns
Gr1 =
2*A + B
Gr2 =
2*A - (d*(Ra^4 - Ri^4))/2
Is there a way to convince MATLAB to return
Gr2 = 2*A - B
in the second case without workarounds? Of course, workarounds like
Gr2 = subs(Gr, [c*(Ra^2 - Ri^2), -d*(Ra^4 - Ri^4)/2], [A, -B])
or
Gr1 = subs(Gr, [(Ra^2 - Ri^2),(Ra^4 - Ri^4)], [A/c,-2*B/d])
Gr2 = subs(Gr, [(Ra^2 - Ri^2),(Ra^4 - Ri^4)], [A/c,2*B/d])
will return the desired result, but I have a much more complex expression to substitute and can't work with this subs()-behaviour. I would have expected MATLAB to automatically simplify the respecive equations.

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采纳的回答

Philip Caplan
Philip Caplan 2015-4-29
As you mentioned, "subs" should return "Gr2 = 2*A -B". I have notified the development team here at MathWorks about this behavior and they will hopefully resolve this in a future release.

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