# Simplify does not simplify

I have this expression

Simplify[2 wb Z - 2 w xiR^4 Zb -
2 xiR^2 Sqrt[(wb Z - w xiR^4 Zb)^2/xiR^4]]


which is actually zero when just calculated by hand, with no assumptions needed...

But Simplify just give the expression back within the brackets. Why is this?

Here in my case, xiR, w, wb, Z and Zb are complex-valued, A is real.

• There is an implicit assumption you've made. In particular, your simplification relies on xiR \[Element] Reals and an assumption about the positivity of (wb Z - w xiR^4 Zb). – evanb Apr 28 '15 at 17:45

Simplify[PowerExpand[