# Simplify and global assumptions

I wanna make global assumptions

 \$Assumptions = A \[Element] Reals && B \[Element] Reals


Then I wanna simplify

 FullSimplify[Re[1/(A + B)]]


I get output

 Re[1/(A + B)]


I wanna get output

 1/(A + B)


What is the problem?

• Learn about ComplexExpand; that's what you are probably looking for... – Henrik Schumacher Jan 12 '18 at 16:12
• Add the assumption A + B != 0? Sometimes the transformations used by Simplify balk if the result could be nonnumeric, or so it seems to me. – Michael E2 Jan 12 '18 at 16:45

Adding the assumption A + B != 0 works with FullSimplify, too:
Simplify[Re[1/(A + B)], A ∈ Reals && B ∈ Reals && A + B != 0]

Sometimes the transformations used by Simplify and FullSimplify balk if the result could be nonnumeric, or so it seems to me.