# Evaluating the real part of an expression

I want to get the Real part of this expression - shouldn't be too hard to evaluate. Why is Mathematica not evaluating but returning the same code?

In == Re[χ/(1 + I ω τ)]
Out == Re[χ/(1 + I ω τ)]


Thank you!

• To directly answer the question as asked: Mathematica has no way of knowing that you intend the symbolic entities $\chi$, $\omega$, and $\tau$ to be real; without further information, such entities might be complex. Dec 7, 2014 at 17:50
• Same as this post Dec 7, 2014 at 17:58

Re can stay unevaluated for numeric arguments: {Re[Log[2 + I]], Re[Sqrt[1 + I]]}
To get around this, try using Re[ComplexExpand[χ/(1 + I ω τ)]].
• To go one step further, you can display the real part by assuming the variables are real (just like ComplexExpand did): Refine[Re[ComplexExpand[\[Chi]/(1+I \[Omega] \[Tau])]],{\[Chi],\[Omega],\[Tau]}\[Element]Reals] Dec 7, 2014 at 8:02