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This problems was reported to Wolfram Support on 30/07/2019.
Support case ID [CASE:4284793].
Wolfram Support acknowledged this behaviour as a bug.
An issue report has been sent to Wolfram developers.
We can reproduce this on
- WolframCloud (12.0 for Linux x86 (64-bit) ) (@rhermans)
- tio.run (11.3 for Linux x86 (64-bit) ) (@rhermans)
- Mathematica 5.2, 8.0.4, 11.3 and 12.0 for Windows 64 (@innaiz, @rhermans)
- Mathematica 11.3. for Mac (@Øistein OP)
Fails:
Simplify[c > f g + h j , f g + h j < c]
FullSimplify[c > f g + h j , f g + h j < c]
FullSimplify[c > f g + h j , Assumptions -> f g + h j < c]
Assuming[f g + h j < c, FullSimplify[c > f g + h j ]]
Doesn't fail
Simplify[c > f g + h , f g + h < c] (* Simpler expressions *)
Simplify[c > f g + h j , c > f g + h j] (* Different order *)