In[2]:= Solve[sqrt(2x-9) == sqrt(4x+3), x]
Out[2]= {{x -> -6}}
But mathematically there is no solution, since sqrt (-21) is not defined. There is a flag that is responsible for this?
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1$\begingroup$ You can consider asking future Mathematica-related questions on Mathematica.SE $\endgroup$ – Szabolcs Apr 20 '12 at 16:09
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By default, Mathematica assumes that all variables are complex numbers, and when working in the set of complex numbers, Sqrt[-21] is well defined.
You can tell Mathematica (version 8) that you are working on the set of reals using
Solve[Sqrt[2 x - 9] == Sqrt[4 x + 3], x, Reals]
which gives no solution. For versions earlier than 8, you need to use Reduce if you want to specify a domain.
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Try using the Reduce equation while requiring x to belong to the Reals. See the Mathematica documentation for an example.
answered Apr 20 '12 at 15:55
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$\begingroup$ @Ticksy You are using incorrect syntax. Please look at some examples in the docs. It's
Sqrt[x], notsqrt(x). $\endgroup$ – Szabolcs Apr 20 '12 at 16:15
