Updates:
Labels: simplify
Comment #2 on issue 4091 by [email protected]: Cannot solve log(x+3) +
log(x-3)=0
http://code.google.com/p/sympy/issues/detail?id=4091
logcombine will only combine arguments that are all known to be
nonnegative; it should probably follow the same logic that is used when
combining powers under Rational exponents and require that all but one be
known in sign.
Here is a case that is solved when the args are known to be nonnegative:
var('n',nonnegative=True)
n
solve(logcombine((log(x + 13) + log(x + 3)-5).subs(x,n)))
[-8 + sqrt(25 + exp(5))]
solve(((log(x + 13) + log(x + 3)-5).subs(x,n)))
[-8 + sqrt(25 + exp(5))]
So for now, the workaround is to use "force=True" with logcombine or solve.
solve(logcombine(log(x - 3) + log(x + 3),force=True))
[-sqrt(10), sqrt(10)]
solve(log(x - 3) + log(x + 3),force=True)
[sqrt(10)]
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