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Let's say I have complex equation

$$ i \frac{dx}{dt} = i x+ (-2ig)^{1/2} $$

$i$ is a complex number and $g$ is just some constant

How do I eliminate the $i$?

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divide both sides by $i$? :) –  Zarrax May 12 '12 at 15:37
    
@Zarrax that still leaves us with one $i$ term, doesn't it? –  Milosz Wielondek May 12 '12 at 16:18
    
You can't. It stays complex equation. –  user2468 May 12 '12 at 16:29
    
You won't eliminate $i$ entirely from the equation, but you'll have $dx/dt - x = $(complex) constant which you can solve using the usual methods, getting a complex-valued solution. There are no real solutions. –  Zarrax May 12 '12 at 17:05
    
@axell don't forget to accept an answer if it answers your question! This way you mark your question for others to see as answered. –  Milosz Wielondek May 13 '12 at 14:18
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1 Answer

Notice that $\sqrt{-2 i}=\pm(1-i)$. Hence the equation becomes $$i \frac{dx}{dt} = i(x\pm \sqrt{g}) \pm \sqrt{g}$$

and it becomes clear that there's no way of completely eliminating $i$ from the equation.

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I don't think $\sqrt{-2i}=\pm(1-i)$ is relevant. I think it can be noticed right away: $$i\frac{dx}{dt}=ix+(-2ig)^{\frac{1}{2}}=ix+i^{\frac{1}{2}}(-2g)^{\frac{1}{2}}$$ Observing that $i$ is not to the same power throughout both sides of the equation automatically convinced me it's not going anywhere. But we may see things differently. :) –  000 May 12 '12 at 19:02
    
@Limitless I was extensively elaborate on purpose, to make it more clearer for the OP. Seeing that each term is has (different powers of) $i$ in it, might mislead an untrained eye to believe that the $i$ can be factored out and eliminated. Having that said, your argument is of course 100% valid, only perhaps not as obvious! –  Milosz Wielondek May 13 '12 at 0:13
    
I did not realize that. Thank you for clarifying. –  000 May 13 '12 at 0:47
    
Sorry for the late reply guys. Thank you for all the responses. So, if I can't solve the equation to get complete real equation, can I at least bring out the $$i$$ from the square root? I mean the complex number without the fractional power outside the bracket? I was thinking of using Binomial expansion but the fractional power made me stuck –  axell May 16 '12 at 3:51
    
@axell yes, you can. Just like I showed in the above answer. –  Milosz Wielondek May 16 '12 at 11:25
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